Prime Minister David Cameron has called on the United Nations to "ramp up" pressure on the regime of Syrian president Bashar Assad, after international peace envoy Kofi Annan announced he was stepping down.
Mr Cameron said that the resignation of the former UN secretary general showed that the Annan plan, which called for a ceasefire in Syria to pave the way for talks on a political settlement, has not worked.
Speaking in Geneva, Mr Annan cited "finger-pointing and name-calling" in the 15-nation UN Security Council as one of the reasons for his decision to step down.
Russia and China have blocked British-backed resolutions in the Security Council to threaten the Assad regime with global sanctions if it fails to halt the bloodshed in the 17-month uprising.
There was little sign of progress on the issue in Downing Street talks between Mr Cameron and Russian president Vladimir Putin, after which the two leaders said only that Moscow and London would continue dialogue.
Speaking to Sky News shortly after the announcement of Mr Annan's resignation, Mr Cameron said: "Frankly it shows that the current process isn't working. The Annan plan - he has worked very hard at it, but it hasn't worked, because we've got this appalling bloodshed, we've got this slaughter.
"I think what we need to do is actually ramp things up. We need to pass resolutions at the United Nations to put further pressure on Syria. I want to see them under so-called Chapter VII (of the UN Charter), so they have full legal backing of the UN - sanctions, travel bans, asset freezes, all the steps we can take to add to the pressure on the Syrians, and also a very clear warning to them that if they use chemical or biological weapons there will be the most severe consequences to them of all."
Mr Cameron played down the prospect of military intervention by the international community, telling Sky News: "Syria is not Libya. It's a different situation. I don't think military approaches are right in this case. I've thought very carefully about that. But I think there's a lot more pressure we can put on and there's a lot we can do, for instance, to help the Syrian opposition with advice and support and I think we can perhaps do more on that."
Mr Cameron defended his decision to host Mr Putin for talks at Number 10, at which Syria topped the agenda.
Speaking after the talks, Mr Cameron pointed to differences in the British and Russian approaches to the Syrian crisis, adding: "We both want to see an end to that conflict and a stable Syria. We will continue to discuss with our foreign ministers how we can take this forward."
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