Syrian troops travelling on tanks, armoured personnel carriers and buses have moved into two northern towns.Security forces were gathering in Khan Sheikhun and Maarat al-Numan, a town of 90,000 between Damascus and Aleppo.
UN Secretary General Ban Ki-moon has urged Syria to halt the bloodshed and "stop killing people".
The UN says that at least 1,100 people have died as the government has violently sought to quash protests over the past three months.
State television said late on Thursday that President Bashar al-Assad's cousin, Rami Makhlouf, intends to give up his business interests - which span telecommunications, construction and oil - and donate his profits to charity.
"As for his businesses, they will be directed so that they [...] create jobs and support the national economy. He will not enter into any new project that (brings) him personal gain," the television said of the tycoon.
The BBC's Jim Muir, reporting from Beirut, says that for someone like Mr Makhlouf - who is part of the elite Syrian inner circle of power - to make such a declaration it is clearly a sign that the regime feels under mounting pressure.
Mr Makhlouf is one of a number of Syrian officials on the European Union's sanctions list, which includes asset freezes and travel bans.
President Assad is facing the gravest threat to his family's 40-year ruling dynasty, as unrest that first erupted in the south of the country has now engulfed the north - near the border with Turkey - and is threatening to spread eastwards towards its border with Iraq.
State television has shown pictures of troops moving into the town of Maarat al-Numan, which sits along the main road between Damascus and Aleppo. The town is some 40km (25 miles) south-east of Jisr al-Shughour.
It said the army had also moved to Khan Sheikhoun, just to the south, to prevent what it called "armed terrorist organisations" from cutting the highway.
Officials have said they are planning a "limited military operation" in Maarat al-Numan to restore security there.
A Syrian human rights activist, Mustafa Osso, is quoted as telling the Associated Press that at least 300 people - mainly men over the age of 16 - were being detained daily. There were also reports of troops opening fire on the outskirts of Maarat al-Numan but not of casualties.
According to the Syrian Observatory for Human Rights, thousands of residents were fleeing from the towns as troops approached.
One eye-witness told BBC Arabic there were "no more than 7,000 people" left in the city of Maarat al-Numan, which had a population of 90,000 people.
Turkish officials are reported to be preparing to send food, clean water, medicine and other aid to thousands more stranded on the Syrian side.
The plan was announced by Turkish Foreign Minister Ahmet Davutoglu after he met with an envoy sent by President Assad, AP reports.
"We have taken precautions and humanitarian aid will be supplied for around 10,000 people who are waiting on the Syrian side of the border," Mr Davutoglu said.
In its report published on Thursday, the UN said that the use of live ammunition against mostly unarmed civilians had killed around 1,100 people.
Syrian rights groups put the overall death toll in Syria at 1,297 civilians and 340 security force members.
UN investigators, who were refused access to the country, believe that as many as 10,000 people have been detained. The authorities appear to have denied civilians the right to food and medical care by laying siege to towns and preventing supply deliveries, it adds.
UN High Commissioner for Human Rights Navi Pillay has repeatedly appealed to Syria to let in a humanitarian team make a proper assessment, but the Syrian authorities have refused.
Meanwhile, Syria has called on the people of Jisr al-Shughour to return, three days after an army attack restored government control there.
More than 8,000 Syrians have fled from the north-western town into Turkey in the past week to escape military operations, which the government says are aimed at tackling "terrorist organisations".
Officials said the city of around 100,000 people - which has been the focus of large anti-regime demonstrations - was returning to normal, but that army units were still pursuing "militants" through the hills around the town.
Damascus says some 120 security personnel were killed by "armed gangs" in the town on 6 June, and on Wednesday state TV said a "mass grave" containing three bodies had been found.
Refugees arriving in Turkey describe the regime's operation in the northern mountains as a "scorched-earth" campaign, and Syrian soldiers who deserted have said they were forced to commit atrocities.
Syria has prevented foreign journalists, including those from the BBC, from entering the country, making it difficult to independently verify reports from there.
European powers are campaigning for a draft UN Security Council resolution condemning the crackdown, but they face opposition from Russia and China, both of whom wield veto powers and object to UN action against President Assad.