The US and Britain have warned the mainstream western-backed Syrian opposition that they will stop supporting them if they fail to send a delegation to next week's Geneva peace conference.

According to a senior official of the Syrian opposition coalition (SOC), Washington and London have made it clear they must take part in talks with President Bashar al-Assad's government.

"The US and UK are telling us you need to go to Geneva," the opposition official said in London. "France is asking us to go but saying we are with you whatever your decision is. This is the same as the Saudi and Turkish stance.

"They [the US and UK] are making it very clear they will not continue to support us the way they are doing now and that we will lose credibility with the international community if we do not go."

The behind-the-scenes tensions burst into public view as the US and Russia jointly called for local ceasefires between the Syrian government and rebel forces. But with expectations for a breakthrough in Geneva so low, it is not clear what if anything will be done to alleviate the plight of the millions of refugees displaced by the conflict.

Britain's foreign secretary, William Hague, urged Russia to drop its opposition to a UN security council resolution backing immediate and unfettered humanitarian access throughout Syria.

He conceded that Britain's provision of £500m in aid was not sufficient. "There are now sickening reports of innocent people dying from malnutrition," Hague said.

John Kerry, the US secretary of state, and Sergei Lavrov, his Russian counterpart, discussed possible prisoner exchanges and the opening of humanitarian corridors to deliver aid. Lavrov said the Assad regime had indicated it was ready to allow access for aid, citing the embattled Damascus suburb of East Ghouta.

The SOC was asked to submit the names of its delegation to the Geneva II conference by the end of November but failed to do so. It has said it will reach a final decision by the end of this week but faces deep internal divisions and the threat of mass resignations.

Its failure to attend the talks would be a huge embarrassment for the UN, the US and Britain and provide further evidence of international disarray in handling the Syrian crisis, which has already cost 126,000 lives since it erupted in March 2011.

Hague said it was vital that the Syrian government and opposition met to agree "a transitional governing body in Syria with full executive powers, formed by mutual consent, to meet the aspirations of the Syrian people".

But it is hard to see how progress is possible when the opposition demands that Assad must go and cannot be part of any transition, while the Syrian leader is adamant he will not step down.

Acknowledging this, Hague told MPs: "No one should underestimate the difficulty of the negotiations ahead. But we will not give up on diplomacy as the route to stopping the appalling bloodshed, nor will we waver in supporting the moderate Syrian opposition, for if there is only a murderous regime on the one side and extremists on the other, then there can be no peaceful settlement in Syria."

He said the SOC president, Ahmed al-Jarba, "has always said it is his intention to attend the Geneva negotiations. His difficult task is to persuade the rest of the moderate opposition to agree to that, at a time when their towns, villages and homes are under relentless attack.

"The national coalition is expected to make a final decision at their general assembly this Friday. We urge them to attend, and to put the spotlight on the Assad regime's responsibility to end this terrible conflict."

Kerry said on Sunday he was confident the coalition would be at the talks, and hinted at a backlash from its supporters if it failed to attend. "I think they understand the stakes," Kerry told reporters in Paris. "But I'm not going to get into consequences other than to say it's a test of the credibility of everybody, and it's why I am confident that they will be there. Because I think they understand that."

The US and UK suspended their non-lethal aid to the opposition last month when supplies were captured by Islamist fighting units in a raid on a warehouse on the Turkish-Syrian border. Hague said aid would resume "as soon as we are satisfied that conditions on the ground allow the military council to take safe delivery of our equipment".

The foreign secretary also said that Iran, a close ally of Assad, could attend the talks if it was prepared to play a constructive role. He called on Tehran to send stronger signals to that effect. Russia is pushing hard for Iran to be invited.

The opposition official made clear that participation could not be taken for granted, but questioned whether the threat to end support was credible. "If the British and Americans say that if we don't go to Geneva they will abandon us, it doesn't mean they will actually do it," he said. "They have a brutal dictator on one side and al-Qaida on the other, so who will they deal with if not with us?"