Britain said on Sunday that foot dragging by President Bashar al-Assad over a United Nations-Arab League peace plan for Syria will trigger fresh pressure on him at the UN Security Council and possibly increase foreign governments' financing of the Syrian opposition.

Foreign Secretary William Hague issued the warning to Assad in comments to journalists before joining a "Friends of Syria" conference, that has brought together 75 countries, mostly Western and Arab states, in Istanbul.

Assad has accepted the peace plan proposed by UN-Arab League envoy Kofi Annan, but his forces continued an artillery and mortar bombardment of a pro-opposition neighbourhood in the city of Homs on Saturday.

"The Assad regime has said they accepted the Annan plan but they haven't done anything about it. The fighting continues," Hague said.

"Increased financial support for the opposition will be one of the consequences of the Assad regime playing for time in implementing the Annan proposals," he said.

"One of the messages from this conference is that it will be important to implement the Annan plan, and there isn't an indefinite amount of time to do that before we here, the 'Friends of Syria,' need to go back to the United Nations Security Council or intensify funding for the Syrian opposition.

"We do need the Assad regime to take action not just words. That does not mean that today we will set an expiry date, but it does depend what they do on the ground."

Britain announced a doubling of non-lethal assistance for the Syrian resistance in recent days, a tactic already adopted by the United States, and one that Turkey has said it will match.

Some Gulf Arab countries have said that more backing should be given to the rebel Syrian Free Army in order to protect civilians caught in Assad's crackdown on the year-long revolt.

Hague said that participants in the Istanbul meeting were discussing what form financing for the opposition should take, adding it was undecided whether it would be a "trust fund", as suggested by some governments.

The British minister noted the efforts by the opposition Syrian National Council to become a more open and inclusive group, saying it had "done better in recent days".