(CNN) -- One day shy of a deadline for Syria to pull its forces from cities across the country, the violence instead seemed to be spreading -- even reaching across two borders, into Turkey and Lebanon.
At least 143 people were killed in Syria on Monday, according to the Local Coordination Committees of Syria, a network of opposition activists. That number was more than twice the death toll from Sunday.
Two others were reported killed over the border in Turkey, as was one near the Lebanese border. In both cases the violence was blamed on gunfire from inside Syria.
The mounting death toll and a government demand to rebels dimmed hopes that the regime would come through on an apparent pledge to withdraw troops. On Sunday, Syria said it had never agreed to a unilateral pullout.
"Should the Syrian government yet again refuse to implement its commitments, make promises and then break them and continue and escalate the killing, then I think it will be clear to all that there isn't yet prospect for diplomatic solution. We still hope that that's possible, we still want to give that a final chance, but I don't think we, or anybody else, are particularly optimistic," Susan Rice, U.S. ambassador to the United Nations, told CNN's John King.
A scathing new report by the group Human Rights Watch, meanwhile, said Syrian forces have summarily executed more than 100 civilians during recent attacks.
While the violence has sent thousands of refugees into Turkey, north of Syria, for months, Monday marked the first known time that bloodshed crossed the border in the conflict.
Two people were killed and 19 others were wounded in clashes between Syrian security forces and the opposition, said a statement from the office of Kilis governorate in Turkey.
The incident began when injured people were trying to cross the border into Turkey, said Selcuk Unal, a Turkish Foreign Ministry spokesman.
Some people "left the camp to get the wounded people," and shots were fired at them and at the camp, he said.
A video apparently filmed by camp residents and uploaded to YouTube showed frantic Syrians carrying a gravely wounded man on a board. Another showed a man, apparently dead, lying in the back of a Turkish police van. A third showed several apparent bullet holes in a white housing unit.
Earlier, Syrian rebels and government forces clashed at the Syrian customs gate Bab el Salama less than a kilometer away, and six Syrian customs and security members were killed, according to the opposition Syrian Observatory for Human Rights.
A Turkish village official, who asked not to be identified because he wasn't authorized to speak to the international media, spoke to CNN by telephone as he drove wounded Syrians in his car from the border fence to a hospital.
"I myself carried six people in my car; my friend carried two," the man said. "There are no ambulances here. We are trying to help the people trying to come through the border ourselves."
Turkey contacted the Syrian Embassy in Ankara to demand that Syrian forces immediately cease fire along the border, Unal said.
"Syrian citizens who have escaped from the violence of the current Syrian regime and who have sheltered in Turkey, are under full guarantee of Turkey," the Turkish Foreign Ministry said in a statement. "We will implement necessary measures in case of recurrence of such incidents."
Syria's state-run news agency SANA had no immediate mention of the incident.
Meanwhile, at the western border with Lebanon, a cameraman with Lebanon's Al-Jadeed television network was killed and a reporter was wounded, according to Lebanese state-run news agency NNA.
Al-Jadeed said the cameraman, Ali Shaaban, "was killed after the news crew's vehicle came under fire by Syrian army troops while on Lebanese soil at the Northern Lebanon/Syria border."
Ahmed Wehbe of Al-Jadeed told CNN the crew was filming a report in Wadi Khaled "when they came under heavy fire from various sides from inside the Syrian borders."
The reporter and a second cameraman had to crawl through fields for two hours until they were rescued by residents of Wadi Khaled, Wehbe said.
Al-Jadeed is traditionally supportive of the regime of Syria's President Bashar al-Assad. But Wehbe said the network blames the Syrian military for the attack "because that area is under the full control of the Syrian military."
Lebanese Prime Minister Najib Mikati tweeted that the incident will be investigated, and said Lebanon deplores and condemns the shooting from the Syrian side of the border. Lebanon will inform Syria of its condemnation, demanding that perpetrators are held to account, Mikati said.
The New York-based Committee to Protect Journalists similarly denounced the attack, saying the situation "is taking yet another dangerous turn as the violence spills beyond Syria's borders, and journalists working in neighboring nations are placed at risk."
SANA said the incident at the Lebanese border took place "when the Syrian border checkpoint came under heavy gunfire from terrorist armed groups." Throughout the more than year-long uprising, Syria has routinely blamed violence on such groups.
A checkpoint came under attack by terrorists who were trying to infiltrate Syria, SANA reported. The report also expressed condolences to the cameraman's family and colleagues, and to Al-Jadeed.
Inside Syria, deaths were reported Monday in Hama, Idlib, Deir Ezzor, Homs, Daraa, and Aleppo, according to the LCC.
The latest violence came a day after Syrian Foreign Ministry spokesman Jihad Makdissi announced that suggestions Syria had agreed to withdraw its troops from cities on April 10 were "a wrong interpretation," according to SANA.
The regime will not commit to pulling out forces only to have "armed terrorist groups" attack, he said.
Early last week, diplomatic officials said the Syrian government agreed to an April 10 deadline to withdraw troops from cities. The agreement came after Syria said it accepted a peace plan laid out by U.N.-Arab League special envoy Kofi Annan. The envoy is expected to be in Turkey on Tuesday, according to the country's Anadolu news agency.
Makdissi said Damascus has acted in "good faith." And he complained that Annan "has not offered written guarantees to the Syrian government that the armed groups agreed to stop violence, nor has he offered guarantees that Qatar, Saudi Arabia and Turkey will commit to stop funding and arming terrorist groups." Those governments have denied such accusations from Syria.
Rebels, including defectors from al-Assad's forces, have taken up arms, but their strength has often paled in comparison with the better-equipped regime troops.
"We can't drop our guns until the regime withdraws from the cities," Lt. Abdullah Odah of the rebel Free Syrian Army said from Istanbul. "We didn't start the mass murder. The regime started it. It has to stop killing, and then automatically we will stop."
U.S. Sen. John McCain was in Turkey on Monday and met with various opposition leaders, who the veteran senator described as "a group of patriots who need our help as Syrians are being massacred."
Amid the conflict, human rights groups have worked to detail what they describe as horrors being carried out by al-Assad's regime.
The new Human Rights Watch report documents "more than a dozen incidents (of executions) involving at least 101 victims since late 2011, many of them in March 2012," the group said.
"In a desperate attempt to crush the uprising, Syrian forces have executed people in cold blood, civilians and opposition fighters alike," said Ole Solvang, emergencies researcher for the human rights group. "They are doing it in broad daylight and in front of witnesses, evidently not concerned about any accountability for their crimes."
The Syrian government has consistently blamed violence in the country on "armed terrorist groups." But the U.N. and world leaders have said the government is lethally cracking down on dissidents seeking true democracy and an ouster of al-Assad, whose family has ruled Syria for 42 years.
Opposition activists and U.N. Secretary-General Ban Ki-moon's office have accused the Syrian regime of ramping up violence in the days leading up to the withdrawal deadline.
"The timeline for the complete cessation of violence endorsed by the Security Council must be respected by all without condition," a spokesman for Ban said Monday.
More than 600 people have been killed since the Syrian government signaled its intention to adhere to the Tuesday deadline to withdraw its forces from cities, according to opposition activists.
CNN cannot independently verify reports of violence and deaths, as the government has severely restricted access by international media.
The United Nations estimates that the violence in Syria has killed at least 9,000 people. The LCC puts the toll at more than 11,000.
CNN's Josh Levs, Ivan Watson, Yesim Comert, Yousuf Basil, Jim Clancy, Salma Abdelaziz and Samira Said contributed to this report.