The federal government is asking that Canadian citizens in Syria leave the country as soon as possible, as a United Nations official warned that an uprising against President Bashar Assad could lead to civil war.

"Canadians in Syria should leave now by commercial means while these are still available," the Department of Foreign Affairs said in an update message on its website.

"Canadians who remain in the country despite this warning should be aware that the Government of Canada's ability to provide consular assistance may be extremely limited due to restrictions imposed by the Syrian Government."

Ottawa had previously advised against travelling to the Middle East country. It upgraded the advisory on the same day that the UN's senior human rights official lambasted the Syrian regime for using "excessive force" against peaceful protesters.

"The Government of Syria has manifestly failed to protect its population. Furthermore, it has ignored the international community's calls to cooperate with international investigations," UN High Commissioner for Human Rights Navi Pillay said in a statement Friday.

Assad is facing mounting pressure over his harsh and bloody crackdown on civilians who have engaged in anti-government protests.

The violence has prompted sanctions from countries in the European Union, the United States and Canada, among others. But the death toll continues to mount.

"The onus is on all members of the international community to take protective action in a collective and decisive manner, before the continual ruthless repression and killings drive the country into a full-blown civil war," Pillay warned.

More than 3,000 people have been killed in Syria since the rebellion against Assad began in March, Pillay said, including 187 children. The Syrian regime has responded to mass protests by taking "excessive" measures including the shelling of residential neighbourhoods.

"As more members of the military refuse to attack civilians and change sides, the crisis is already showing worrying signs of descending into an armed struggle," Pillay said.

On Thursday, 25 people were killed in clashes between Syrian troops and gunmen who were believed to be defectors from the armed forces, according to the London-based Syrian Observatory for Human Rights.

At least seven people died on Friday when security forces fired on protesters, activists said. The shootings took place at a demonstration in support of a group of army defectors known as the Free Syrian Army.

With files from The Associated Press