Syrian leader Bashar al-Assad is coming under increased international pressure to halt a deadly crackdown on pro-democracy supporters in the country.

Britain and the US have announced looking into the possibility for imposing sanctions on Syria to push it into halting the violence against protesters and respecting human rights.

An estimated 350 people have been killed since the uprising against Assad's 11-year rule began on March 15.

According to a diplomat on the UN Security Council, France, Britain, Germany and Portugal have circulated a draft media statement to the other members of the council, urging them to strongly condemn the violence against peaceful demonstrators.

The matter was discussed at a Security Council meeting on Tuesday afternoon, the diplomat said.

The draft statement supports a call by Ban Ki-moon, the UN secretary-general, for an independent and transparent investigation into the killings in Syria.