Hundreds of displaced Syrians have fled into Turkey after Syrian troops, backed by tanks, approached their makeshift camps along the border.
Witnesses said on Thursday that Syrian troops had massed on the Turkish border overnight, escalating tensions with Ankara as President Bashar al-Assad uses increasing military force to try to crush a popular revolt.
Refugees from the northwestern province of Idlib said armoured vehicles and troops were now as close as 500 metres from the border in the Khirbat al-Jouz area.
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Al Jazeera's Anita McNaught, reporting from the Turkish border village of Guvecci, said that she could see Syrian soldiers from where she was.
"We can see soldiers and armoured trucks just across the border, within view from this refugee camp that we're in," she said.
"We were told at 6:30 this morning, that people here received calls from Syria saying that Syrian troops had moved in with tanks and armoured vehicles and they were clearing the village out."
McNaught also said a building in Syrian territory, that had carried a Turkish flag, was now carrying a Syrian flag and had snipers based on the roof.
"We can see men carrying rifles standing on the building and we're being told that those are snipers up there, on patrol."
Syrian armoured personnel carriers were visible on a road running along the top of the hill, and machine-gun fire was heard although it was not clear who the troops were firing at.
Activists said Syrian security forces had arrived in the border town of Khirbet Al-Jouz, and people that fled to Turkey said there were about 20 tanks in the village.
Our correspondent said that while there was no official statement from Turkey, the government had brought in buses for those refugees that wanted to evacuate the area.
Several hundred people broke through the barbed wire marking the frontier between the two countries and were seen advancing into Turkish territory on a road used by Turkish border guards, a few kilometres from Guvecci.
They were flanked by Turkish paramilitary police vehicles and minibuses, called apparently to ferry the refugees to tent cities the Turkish Red Crescent has erected in the border province of Hatay.
Another group of several hundred people was seen further down the same road, walking towards the Turkish security forces vehicles.
At the weekend, the Turkish Red Crescent announced it had begun providing urgent humanitarian aid to those massed on the other side of the border.
More than 1,300 civilians have been killed and some 10,000 people arrested, according to Syrian human rights groups, in the crackdown that has seen troops dispatched to crush pro-democracy protests across Syria since March.