Presidential elections in Syria will be a "parody of democracy", UK Foreign Secretary William Hague has said.
Mr Hague said Syria's government had an "utter disregard" for life, and President Bashar al-Assad decision to call an election for 3 June "disgusted" the international community.
The foreign secretary also announced the Syrian opposition would have its diplomatic status in the UK upgraded.
Syria's three-year conflict has left some 150,000 people dead.
The UK is continuing to push for President Assad to stand down, but he has sought a third seven-year term in the elections.
Mr Hague hosted a meeting of the The Friends of Syria group - made up of Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates, the UK and the US - in London.
Afterwards, he said: "We are of course united in our disgust and anger at what's happening in Syria and the ruthless utter disregard for human life."
The group agreed a short communique criticising the decision to hold an election at a time when millions of people were displaced and the bloodshed was continuing.
Mr Hague called on the whole international community to "reject these illegitimate elections", saying: "We've also agreed unanimously to take further steps to... do everything we can to hold the Assad regime accountable for the terror it is perpetrating."
He promised the UK government would increase its humanitarian efforts, with £30m of extra funding.
A Foreign Office spokeswoman said this money would be used for "a range of different things including helping the opposition and supporting regional sustainability".
In a separate media briefing, US Secretary of State John Kerry commented on France's claim that the Assad government has used chemical weapons at least 14 times since October.
He said he had seen "raw data that suggests that there may have been... a number of instances in which chlorine has been used in the conduct of war".
Mr Kerry added: "If it has, and it could be proven, then that would be against the agreements of the chemical weapons treaty and against the weapons convention that Syria has signed up to."
"Out of today's meeting, every facet of what can be done is going to be ramped up - every facet. That includes political effort. It includes aid to the opposition. It includes economic efforts, sanctions," he said.
Friends of Syria was set up in 2012 in response to moves by Russia and China to block UN resolutions against the Assad government.
The meeting comes days after UN special envoy Lakhdar Brahimi stepped down over lack of progress in ending the crisis.