Tanks deploy around Rastan in Syrian protest crackdown

The protests, which began on 15 March, represent the biggest challenge to a four decade-long rule by the Assad family in one of the Arab world's most repressive and tightly controlled countries.

Large numbers of people have been reportedly arrested in recent days amid reports of tear gas, mass arrests, and live bullets being employed to break up protests.

Some 2,843 people are confirmed as detained, although campaigners say the total could be nearer to 8,000.

They say detainees have been subjected to torture, reporting that in the western city of Zabadani alone more than 30 activists were released who had been brutally tortured - many having had their fingernails pulled out by pincers.

The US has denounced the crackdown as "barbaric" and UN Secretary General Ban Ki-moon has called on President Bashar al-Assad to end a six-week clampdown on opposition supporters.

In his phone conversation with Assad, Ban "reiterated his calls for an immediate end to violence against and mass arrests of peaceful demonstrators in Syria, and for an independent investigation of all killings that happened during the protests", said a statement from his office.

He also "called on President Assad to immediately grant access to the United Nations in order to assess the humanitarian needs of the affected civilian population".

Human rights groups say about 560 people have been killed across the country in anti-government protests.

Foreign journalists are barred from Syria, and reports cannot be verified.

On Tuesday, Syrian tanks and troops reportedly surrounded the coastal city of Baniyas while Deraa, the southern city where the protests kicked off in mid-March, has been under siege since the weekend.

On Wednesday, there were fresh reports of gunfire and mass arrests there.

The army is still not allowing anyone into the city, and supplies of food and medicine are running low, residents have been reported as saying.

Also on Tuesday, a late-night demonstration by 1,000 students at Aleppo University - in Syria's second city - was broken up by security forces using tear gas and firing live bullets. No injuries were reported, but dozens of students are being arrested, their mobile phones and laptops confiscated, the sources say.