Syrian troops and gunmen loyal to President Bashar al Assad have arrested 70 people and set houses on fire after tanks entered a town near the Turkish border, according to witnesses.
It comes as the Foreign and Commonwealth Office updated its advice to British nationals in Syria, saying people should "leave now by commercial means whilst these are still operating".
The FCO had been warning against travelling to the country since 24 April.
A statement said: "Those who choose to remain in Syria, or to visit against our advice should be aware that it is highly unlikely that the British Embassy in Damascus would be able to provide a normal consular service in the event of a further breakdown in law and order and increased violent civil disorder."
Amid the unrest, there are now reportedly at least 10,000 Syrian refugees in camps in Turkey.
The latest anti-government demonstrations took place on Friday in many parts of the country.
They included the southern province of Deraa, the city of Hama north of Damascus and in suburbs of the capital itself.
Activists said at least 19 people were shot dead during the protests.
Another two were reportedly killed whilst trying to tear down posters of President Assad and his father.
Demonstrations first broke out in March in protest at the 41 years of rule by the Assad family.
A lawyer living in the border town of Bdama, in the Jisr al Shughour region, said he saw the troops arriving in the town.
"I counted nine tanks, 10 armoured carriers, 20 jeeps and 10 buses."
"I saw (pro-Assad gunmen) setting fire to two houses," he added.
Another witness reported seeing government troops burning crops.
Actress Angelina Jolie visited one of the Turkish refugee camps on Friday in her capacity as a United Nations (UN) goodwill ambassador.
Officials said the Syrians are given three meals a day, medical care, hot water and access to washing machines and televisions.
The Syrian Observatory for Human Rights said nearly 1,300 civilians and 340 security force members have been killed in the country since March.
Authorities in Syria have blamed the unrest on armed groups and Islamists, backed by foreign powers.
Most international journalists have been banned from reporting from the country, so accounts from activists and officials can not be verified.