Syria accused the United States of meddling after the US ambassador visited the flashpoint city of Hama, where hundreds have fled fearing a crackdown ahead of anti-regime demonstrations on Friday.
"The presence of the US ambassador in Hama without previous permission is obvious proof of the implication of the United States in the ongoing events, and of their attempts to increase (tensions), which damage Syria's security and stability," the foreign ministry said in a statement.
"Syria warns against such irresponsible behaviour and stresses its determination to continue to take all measures that will bring back calm and stability to the country," it added after Robert Ford's visit.
Hundreds of people have fled Hama ahead of planned demonstrations Friday under the banner of "no to dialogue" with President Bashar al-Assad's regime, activists said.
They were headed for Salamiyah, some 30 kilometres (18 miles) to the southeast.
Syrian authorities have been trying to quell protests in Hama, traditionally a centre of opposition to the central government, and had positioned tanks on the main entrances to the city, except in the north.
The Syrian Observatory for Human Rights said that about 100 families -- or 1,000 people in total -- had left Hama, where it said Syrian troops had killed 25 civilians since Tuesday.
Observatory chief Rami Abdel Rahman said two civilians were killed on Thursday at Hama.
"Security forces shot them in the legs and then ran them over in their vehicle. They were fatally injured and died on the way to hospital," he said.
A senior US official, speaking on condition of anonymity, said Ambassador Ford visited the tense city to "make contact" with opposition leaders.
Hama has been a symbol of opposition since the 1982 crackdown on a revolt by the banned Muslim Brotherhood against then-president Hafez al-Assad, father of the present leader, in which some 20,000 people were killed.
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Al-Watan newspaper, which is close to the regime, said the situation in Hama was calm and that barricades erected in the streets by protesters had been dismantled.
The authorities had told demonstrators to avoid any confrontations and clear the streets so residents could go to work and to avoid what it called a "last resort" military operation, according to the newspaper.
It also said protesters were calling for the former governor to be reinstated, detained demonstrators to be freed, a pledge that the security forces would not intervene and a guarantee of freedom to demonstrate.
Last Friday, an anti-regime rally brought out half a million people in Hama, according to pro-democracy activists. The security services did not intervene and Assad fired the city's governor the next day.
Human rights activists said that anti-regime demonstrations took place overnight into Thursday in several towns in response to a number of pro-regime rallies held on Wednesday.
Residents of Hama and the central city of Homs staged a general strike ahead of Friday demonstrations, according to Abdel Rahman.
"Dialogue makes no sense if security forces do not pull out of the streets and the regime does not stop its violence against citizens," lawyer Anwar al-Bunni told AFP.
Rights groups say that more than 1,300 civilians have been killed and 10,000 people arrested by security forces since mid-March when the anti-government protests erupted.
UN chief Ban Ki-moon called on the Syrian president to make good on his promises.
"In Syria, meanwhile, the killing continues. This must stop," Ban said in Geneva.
"I call on the Syrian leadership to deliver on its commitments and to allow our UN humanitarian assessment team and the human rights fact-finding mission mandated by the Human Rights Council in.
"It's time to see progress here. We cannot go on like this."