Britain is to recognise the Syrian opposition as a "legitimate representative" of the country's people, William Hague said today.
The Foreign Secretary made the comment as he arrived in Tunisia for talks on the response to brutal repression by Syrian president Bashar Assad's regime.
He said the international community had to "tighten the diplomatic and economic stranglehold" on the Syrian government, amid growing concern over bloodshed in Homs and other cities.
"We will intensify our links with the opposition," Mr Hague said. "I will meet leaders of the Syrian National Council in a few minutes' time, before the main meeting takes place.
"We, in common with other nations, will now treat them and recognise them as a legitimate representative of the Syrian people."
Mr Hague denounced Assad's government as "a criminal regime".
But he indicated that Britain is not contemplating sending arms to the Syrian opposition, as some people have demanded.
"There may well be people who say that, and it reflects the intense frustration that we all feel," he said.
"We have in the European Union an arms embargo on Syria, so of course we will observe that arms embargo in all directions."
Mr Hague said he hoped the Chinese government would change its position.
"I haven't seen that shift in Russia yet," said the Foreign Secretary.
"I think the Chinese government is constantly assessing the position and so I hope... immediately they will change their position, but if not that, then they will steadily do so over time.
"When one looks at things like the report yesterday pointing to crimes against humanity and a continued deterioration in the situation and Syria sliding towards greater chaos and bloodshed, not out of it, then it is very important in Moscow and Beijing for there to be a re-evaluation of the position.
"But in the meantime, it doesn't mean the rest of us sit and do nothing."
Mr Hague said he expected EU countries to agree to tougher sanctions against the Syrian regime at a meeting in Brussels on Monday.
"One of the things I will say to the other countries - the non-European countries here - is that we hope they will step up their sanctions on the Assad regime and tighten a diplomatic and economic stranglehold on the Assad regime.
International leaders gathered in Tunis set to demand that president Assad allow a ceasefire so that help can be got into the worst-affected areas as well as discussing ways to increase pressure on the regime.
Prime Minister David Cameron said he hoped the "Friends of Syria" meeting would find ways to back the opposition and to tighten sanctions - and called on Russia and China to join the condemnation.
Neither country is attending amid anger over their decision to veto a United Nations Security Council resolution backing an Arab League peace plan.
The UN last night appointed its former secretary-general Kofi Annan as a joint envoy with the Arab League on the Syrian crisis.
In a strongly-worded appeal for the world to unite to end the bloodshed, Mr Cameron seized on the death of Sunday Times journalist Marie Colvin as a signal of the regime's intent.
Her killing was not just a tragic loss but "yet another evil act by the Syrian government", Mr Cameron declared, accusing Mr Assad of murdering and maiming children in the crackdown.
"We do need to work hard to work out what more we can do as an international community," he told reporters yesterday, appealing directly to Moscow and Beijing.
"It's absolutely vital that the international community comes together, does this work, sends this message, and I hope that the foreign ministers' meeting in Tunis tomorrow will back that up as strongly as they can."