Hundreds of terrified refugees crossed into Turkey on Thursday to escape an army assault on the border regions, witnesses said.
Protests have grown in northern areas bordering Turkey, following military assaults on towns and villages in the Jisr al-Shughour region of Idlib province to the west of Aleppo that had sent more than 10,000 fleeing to Turkey.
Troops were advancing on a main road leading from Aleppo, the commercial hub, to Turkey, residents said.
On the 100th day of an uprising that has posed the gravest challenge to Assad's rule, soldiers and secret police backed by armoured vehicles set up road blocks on Wednesday along the main road, a major route for container traffic from Europe to the Middle East. They arrested tens of people in the Heitan area north of Aleppo, residents said.
"The regime is trying to pre-empt unrest in Aleppo by cutting off logistics with Turkey. A lot of people here use Turkish mobile phone networks to escape Syrian spying on their calls and have family links with Turkey. There are also many old smuggling routes that people could use to flee," one of the residents, a physician, told Reuters by telephone.
Refugees from the northwestern province of Idlib said armoured vehicles and troops were now as close as 500 metres to the Turkish border in the Khirbat al-Joz area.
A Reuters photographer in the Turkish border village of Guvecci saw three uniformed Syrian soldiers with a machinegun positioned on the roof of a house on top of a hill. Syrian armoured personnel carriers were also visible on a road running along the top of the hill, and machinegun fire was heard although it was not clear at whom the troops were firing.
Growing protests in the north
On Thursday Reuters cameramen said a watchtower which had for the last few days displayed a Turkish flag was now flying a Syrian flag.
Turkish soldiers at the border were wearing helmets for the first time since being deployed earlier this month, Reuters witnesses said.
Sunni Turkey has become increasingly critical of the Syrian president, after previously backing him in his drive to seek peace with Israel and improve relations with the United States, while Assad opened the Syrian market to Turkish goods.
Central neighbourhoods of Aleppo, a largely Sunni city with a significant minority population, has been largely free of protests, in part due to a heavy security presence and a continuing alliance between Sunni business families and Syria's ruling Alawite hierarchy.
But activists said security forces killed one protester in Aleppo on Friday and arrested 218 students at Aleppo University, scene of now daily protests, in the last three days.
In Idlib, rights groups say Syrian security forces have killed more than 130 civilians and arrested 2,000 in a scorched earth campaign to crush dissent in the province. Some 1,300 people have been killed since mid-March, rights groups say.
The United Nations High Commissioner for refugees said that since June 7, some 500-1,500 people had fled daily across Syria's 840-km (520-mile) border with Turkey.
A country of 20 million, Syria is largely Sunni, and the protests have been biggest in mostly Sunni rural areas and towns and cities, as opposed to mixed areas.
Analysts say there is a high risk that Syria, with its mix of Sunni, Kurdish, Alawite and Christians, could slip into war as Assad increasingly relies on loyalist Alawite troops and irregular forces known as 'shabbiha'.
A large part of the Sunni community is resentful of privileges granted to minority Alawites who have also dominated the security apparatus during the 41 years of Assad dynasty rule and are wary of Assad's policy of aligning Syria further with Shi'ite Iran and Lebanon's Hezbollah guerrillas.
Ban says Assad losing credibility
Turkey had warned Assad against repeating mass killings in cities witnessed during the rule of his father in the 1980s. A senior Turkish official said on Sunday that Assad had less than a week to start implementing long-promised political reforms before foreign intervention began, without elaborating.
Syria's Foreign Minister Walid al-Moualem on Wednesday played down any possibility of international intervention against his country. He asked Turkey to reconsider its response to a speech this week by Assad in which Turkish President Abdullah Gul said Assad's promises of reforms were not enough.
In his third speech since the start of the uprising, Assad promised reforms but these were seen by opponents and world leaders as too little, too late and too vague.
Assad issued an amnesty the next day, which human rights lawyers said covered mainly drug dealers, tax evaders and thieves across Syria.
UN chief Ban Ki-moon said Assad was running low on credibility, after his pledges for reform and engaging in dialogue with pro-democray protesters had yet to bear fruit.
"I do not see much credibility (in) what he has been saying, because the situation has been continuing like this way, and how long should the situation be going (on in) this way," Ban told reporters when asked how credible he considered Assad's pledges of reform and other statements to be.