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."