British Foreign Secretary William Hague warned on Sunday that gains made by the regime in the Syrian conflict this week made it harder to organise a peace conference and to make it a success.
He said it was "worrying and depressing" that the so-called Geneva talks were not taking place this month, and repeated his warning that the world must do more to help the people of Syria.
"The regime has gained ground on the ground, again at the cost of huge loss of life and the indiscriminate use of violence against the civilian population," Hague told BBC television.
"That makes the Geneva conference harder to bring about and to make a success. It makes it less likely that the regime will make enough concessions in such negotiations, and it makes it harder to get the opposition to come to the negotiations.
"The way the position on the ground is changing in Syria at the moment isn't helping us bring about a political and diplomatic (solution)."
Asked if he believed the Geneva talks would happen at all, Hague said: "We're working on that. We're in intensive discussions with the US and of course with Russia and the UN about this.
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"But they're not coming together in the next couple of weeks and I find that worrying and depressing."
Hague repeated that Britain had not yet taken a decision on arming the rebels in Syria but confirmed for the first time that there would be a vote in parliament before it did so.
Several MPs had expressed concerns the weapons could fall into the wrong hands.
"There would be a vote one way or the other. There isn't an established procedure for it, but I can't see any reason why it wouldn't be before any such decision was implemented," Hague said.
He added: "People have understandable concerns about the idea of sending arms to anybody in Syria.
"We'd all be very reluctant to do that. On the other hand at the moment people are being killed in huge numbers while the world denies them the means to defend themselves."
Although the regime has seized back control of the entire Qusayr region near the Lebanese border, Hague said President Bashar al-Assad must be "realistic" about the need for a political solution.
The regime must "understand that they are never going to be able to undertake the total military conquest and subjection of their country by force -- that that would leave a permanently unstable country and region", he said.