Foreign Secretary William Hague has warned of an escalation of violence and bloodshed in Syria over the coming days.
Forces loyal to President Bashar Assad are currently carrying out a major assault on the rebel stronghold of Aleppo.
Heavy artillery, planes and a helicopter gunship are reportedly bombarding rebel positions in the country's largest city in a battle that could decide the fate of the 17-month uprising.
Mr Hague warned that President Assad's reliance on violence, rather than negotiations, could lead to "a collapse in all authority" in the country.
In a TV interview he said: "There has been fighting in Aleppo over recent days, but it does seem that the Syrian armed forces have been building up to something bigger.
"We are likely, sadly, to see even more fighting, more bloodshed, probably a bigger flow of refugees over the coming days.
"This is, I'm afraid, the situation we warned about for a long time - that there would be a collapse in Syria and an even more deadly conflict if we didn't bring enough pressure to bear on the Assad regime to implement the peace plan of Kofi Annan.
"Our Security Council resolution was vetoed. We won't give up on the diplomatic work, but given this situation, we will step up our humanitarian assistance but also our practical help to the opposition in Syria, who are in such a desperate situation."
Mr Hague said the "prime responsibility" for the conflict, which has claimed at least 19,000 lives, lay with the Assad regime.