Syrian forces fired across the border into a refugee camp in Turkey, wounding at least five people as a UN-brokered plan to end more than a year of violence this week all but collapsed, authorities said.
Syrian activists said two people were killed, but the reports could not be immediately confirmed.
The Syrian soldiers were believed to be firing at rebels who tried to escape to the refugee camp after attacking a military checkpoint, according to the Britain-based Syrian Observatory for Human Rights, citing a network of sources on the ground.
Turkey shelters thousands of refugees who have fled Syria as the government tries to crush a revolt against President Bashar Assad. The UN estimates some 9,000 people have been killed in Syria since March 2011, when the uprising began.
Today's shooting bolstered fears that the uprising could spark a broad conflagration by sucking in neighboring countries. There have been similar cross-border attacks into Lebanon, although Monday's shooting was believed to be the first inside Turkey.
The incident began at about before dawn on Monday when rebel fighters attacked Syrian soldiers manning a checkpoint near the Turkish border, said Rami Abdul-Rahman, a spokesman for the Observatory.
Six soldiers were killed in the ambush, Abdul-Rahman said. The troops then kept firing as eight wounded rebels escaped to the camp that is just over the border in Turkey, sending bullets whizzing across the frontier into the camp, he said.
According to the Observatory, the shooting wounded five people in the camp, which is near the town of Kilis in the southern province of Gaziantep. The Observatory reported that two people later died of their injuries, but that could not be immediately confirmed.
The governor of Gaziantep province, Yusuf Odabas, said five people were wounded: three Syrians, one Turkish translator and one Turkish policemen. The translator had entered the camp to try to help calm an anti-Assad protest, he said. The governor said Turkish military forces did not return fire.
Turkey hosts some 24,000 Syrian refugees, including hundreds of army defectors, and has floated the idea of setting up a buffer zone inside Syria if the flow of displaced people across its border becomes overwhelming.
Parts of the southern Turkish region near Syria are informal logistics bases for rebels, who collect food and other supplies in Turkey and deliver them to comrades inside Syria via smuggling routes.
The Syrian uprising began last year with mostly peaceful protests against the Assad regime, a family dynasty that has ruled the country for four decades. But in the face of a relentless military assault on protests, the opposition has become increasingly militarized.
Now, the uprising resembles an armed insurgency, and there are fears the country is spiraling toward civil war. International envoy Kofi Annan brokered a cease-fire that was supposed to begin Tuesday, but the plan is in tatters.
Syrian troops were meant to pull out of population centers by Tuesday morning, but the government on Sunday introduced a new demand — saying it cannot withdraw without written guarantees from opposition fighters that they will lay down their arms. Syria's main rebel group rejected the government's demands.
Naci Koru, Turkey's deputy foreign minister, said the deadline for the withdrawal has become "void at this stage," state-run TRT television reported.
Annan is scheduled to visit to one of the refugee camps in Hatay province, bordering Syria, on Tuesday afternoon, Turkey's Foreign Ministry said. Annan's office confirmed the trip to Turkey.
"Annan's one-hour visit to Hatay tomorrow is critical, he will see the situation himself," the TRT quoted Koru as saying on Monday.
Annan has been on a serious diplomatic push to rally support for his cease-fire deal. The international community, which so far is unwilling to contemplate military intervention, has had little leverage over Syria.
But Iran, Russia and China have been Assad's strongest supporters. Annan already has traveled to Russia and China, and he was expected in Tehran on Tuesday.
On Monday, Russia was hosting Syrian Foreign Minister Walid Moallem.
It is not clear whether Moscow will try to pressure Syria to comply with the cease-fire plan, though Russia said Monday it may send its observers to Syria as part of a potential UN monitoring mission.
Also on Monday, Human Rights Watch said Syrian forces have summarily executed more than 100 people, most of them civilians.
The New York-based group said it has documented the killings of 85 civilians, including women and children, and the summary executions of at least 16 wounded or captured opposition fighters.
"In a desperate attempt to crush the uprising, Syrian forces have executed people in cold blood, civilians and opposition fighters alike," said Ole Solvang, a researcher for the group. "They are doing it in broad daylight and in front of witnesses, evidently not concerned about any accountability for their crimes."
The New York-based group said it only included cases corroborated by witnesses, but has received many more reports of similar incidents.
The group said it documented several cases of mass executions in March in the cities of Homs and Idlib, two centers of the uprising. This includes the killing of 13 people in a mosque in Idlib, the executions of 25 men in a raid of the Sultaniya neighborhood of Homs, and the killing of 47 people, mainly women and children, in three other areas of Homs, the group said.
Two witnesses describing the March 11 killings in Idlib said the Bilal mosque in the city had been used as an initial collection point for those killed and wounded in an army raid. When relatives came to identify the dead, several were led by soldiers out of the mosque, blindfolded and lined up against a wall. More than a dozen soldiers opened fire, killing at least 13 people, the witnesses said.
The Syrian government typically does not comment on such reports.
The allegations came as opposition activists reported that Syrian forces pressed ahead with raids and shelling attacks on the towns of Tel Rifaat in the northwest and Muhassan in the east of the country on Monday.