WASHINGTON — Britain will boost support for Syria's opposition and seek tougher sanctions if government forces do not adhere to a ceasefire, Foreign Secretary William Hague warned Wednesday.

Hague spoke at a meeting of the Group of Eight powers in Washington as Syria said that it would cease operations against rebel fighters from Thursday -- the date set by peace envoy Kofi Annan to halt more than a year of bloodshed.

"Our pressure on the regime -- its campaign of murder, torture and oppression -- must be intensified if Kofi Annan has not succeeded," Hague told reporters.

If Syria respects the ceasefire, Hague called for "very urgent work" including monitoring to ensure that violence does not resume.

"But if there isn't a ceasefire or if a ceasefire is not upheld over the coming days, then in the view of the United Kingdom, we will want to return to the Security Council in a new attempt to obtain a resolution on Syria, we will intensify our support for the opposition -- the Syrian opposition -- and we will seek strong sanctions," Hague said.

The Group of Eight comprises major industrial powers including Russia, which has clashed with Western and Arab nations by supporting Syrian President Bashar al-Assad both through diplomatic support and arms shipments.

Hague acknowledged "many differences" with Russia but said that Moscow has assured that it has put pressure on Syria in recent days.

"So I think on this point of achieving a ceasefire and launching a political process, there is unity in the G8 and in the UN Security Council," Hague said.

"It would be harder to achieve a resolution in the event of a ceasefire not being enforced. But in that situation, Syria would have defied the whole of the UN Security Council and the whole of the world," he said.