Thousands of Palestinian refugees have been forced to flee a camp in the Syrian port of Latakia amid shelling by government troops, the UN says.
A spokesman for the United Nations Relief and Works agency (Unrwa) told the BBC that more than 5,000 of the 10,000 refugees were on the move.
He said at least four people had died, urging immediate access to the site.
Some 30 people have reportedly died in Latakia in a three-day military attack. Syria says it is tackling gangs.
On Monday, there were also reports of a clampdown in the capital Damascus, with people being arrested randomly in the Jobar district.
Turkish Foreign Minister Ahmet Davutoglu warned Syria that military operations against protesters must stop “immediately and unconditionally”.
Ankara, a former close ally of Damascus, has been increasingly frustrated with its crackdown of the unrest.
The Palestinian authorities urged the Syrian government to safeguard the lives of Palestinian refugees.
In Washington, White House spokesman John Carney said that Syrian President Bashar al-Assad must “cease the systematic violence, mass arrests and the outright murder of his own people.”
More than 1,700 people have reportedly died and more than 30,000 have been detained in the five-month uprising against President Assad’s rule.
On Monday, Unrwa spokesman Christopher Gunness told the BBC that more than 5,000 Palestinian refugees from the camp in Latakia’s al-Ramel district and surrounding areas had already fled.
“We have no idea where these people are, we have no idea how many of them are wounded, are dying, are elderly, are women, are children,” he said.
He said that at least four people were confirmed dead and nearly 20 were injured.
Mr Gunness added that some refugees had been told by the Syrian government to leave the camp.
He described the situation in the camp as “alarming”, calling on Damascus to grant Unwra immediate access to the site to establish “what is going on”.
However, similar appeals in the past have been ignored in the past by the Syrian government, the BBC’s Jim Muir in Beirut says.
Mr Gunness also said that reports from the site suggested that the Syrian military was using tanks and gunboats.
A Syrian military official on Monday denied as “absolutely baseless” reports that gunboats had fired on Latakia, Syria’s official Sana news agency reported.
Activists on the ground said later on Monday that Syrian troops had taken control of the Ramel district.
The crackdown on Latakia began on Saturday, a day after mass anti-government protests in the city.
On Sunday, activists said Syrian warships had joined the attack, firing shells on the city.
One resident of Ramel told the Associated Press news agency: “We are being targeted from the ground and the sea. The shooting is intense. We cannot go out. They are raiding and breaking into people’s homes.”
A UK-based Anglican priest who is visiting his family in Latakia said earlier that the atmosphere in the city was extremely tense.
“[On Sunday] you could hear a lot of shooting and bombing from different parts of the city,” the Reverend Nadim Nassar told the BBC. “The whole city is now shut… the fear is very high,” he said.
Syria’s state TV denied any shelling had happened.
It said the security forces were fighting armed gangs who had set up barricades and were shooting from rooftops.
The government said three members of the security forces were killed and 40 wounded in clashes. It interviewed some of the city’s residents in other places who called on the army to clear out the “terrorist gangs”.
Activists deny that their movement is armed but said at least one officer and a number of soldiers had defected to join the uprising.
International journalists face severe restrictions in operating in Syria, and it is hard to verify reports.
Meanwhile, newspapers in the region have expressed anger about Arab states’ failure to respond to events in Syria.
Latakia has seen many anti-government protests in the past five months.
Syria has come under increased diplomatic pressure in the past week to stop its crackdown on the dissent.
The US has imposed sanctions on Damascus and has said these could be increased, while calling on other countries to follow.
Saudi Arabia, Bahrain and Kuwait have all recalled their ambassadors.
Mr Assad has reiterated promises of political reform, while remaining adamant his government would continue to pursue the “terrorist groups” he has blamed for the unrest.
In recent weeks protesters have also been targeted in Homs, Hama, Deir al-Zour in the east, Deraa in the south and Aleppo and Idlib near Turkey’s border.