Britain and the United States have told the main Syrian opposition that they will stop their support if it fails to send a delegation to peace talks this month, British media reported on Tuesday.
"The US and UK are telling us you need to go to Geneva," an unnamed senior official in the Syrian National Coalition was quoted as saying by the BBC and the Guardian newspaper.
"They are making it very clear that 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."
According to the BBC, the official questioned however whether Britain and the US had any choice in who they dealt with, saying: "What is the alternative?
"They have a brutal dictator who used chemical weapons on one side and al-Qaeda on the other, so who will they deal with, if not with us?"
US Secretary of State John Kerry expressed hope this week that the opposition would attend the so-called Geneva II talks set to open on January 22, saying they were a "test of credibility of everybody" in the conflict.
The National Coalition is divided over whether to attend the talks in the Swiss city of Montreux but is expected to make a decision on Friday.
"We urge it to attend and to put the spotlight on the Assad regime's responsibility to end this terrible conflict," British Foreign Secretary William Hague said on Monday.
The National Coalition official, speaking in London, said other western backers of the opposition were not applying the same pressure as Britain and the United States.
"France is asking us to go but saying that we are with you whatever your decision. That is the same as the Saudi and Turkish stance," the official was quoted as saying.