The coming days are likely to see an escalation of violence and bloodshed in Syria, as the regime of President Bashar Assad unleashes its armed forces for a major assault on rebel stronghold Aleppo, Foreign Secretary William Hague has warned.
Reports from Aleppo - Syria's largest city and commercial hub - said that Assad's forces are using heavy artillery, planes and a helicopter gunship to bombard rebel positions in a battle that could determine the outcome of the 17-month uprising.
Mr Hague warned that the regime's reliance on violence, rather than negotiations, in responding to challenges to Assad's rule, could lead to "a collapse in all authority" in the Middle Eastern country.
And following the Russian and Chinese veto of British-backed resolutions at the UN Security Council and the resignation of peace envoy Kofi Annan, the Foreign Secretary said that it may require a deterioration in circumstances on the ground to trigger a return to diplomatic efforts to stop the violence.
Britain is stepping up provision of non-lethal support, such as communications equipment, to opposition groups and is the second-largest provider of humanitarian aid to refugees fleeing the fighting, but is not supplying arms to the rebels.
Opposition commanders were said to be preparing for a "strong offensive" on Aleppo by government forces, while in the capital Damascus, rebels were coming under attack by regime jets.
Mr Hague told Sky News: "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 since the first pro-democracy protests in Syria, lay with the Assad regime.