The British Prime Minister David Cameron has backed Washington's "candid assessment" that the Syrian regime has used chemical weapons and admitted it left international leaders facing "difficult" questions.

US President Barack Obama has approved arming the Syrian rebels for the first time in response to the development, according to officials.

The Prime Minister, who will speak to the President by telephone later on Friday ahead of the G8 summit, said President Bashar al Assad's use of the weapons against his own people "under our noses" must be addressed.

But the Syrian government has dismissed US statements as "full of lies".

A statement by the Syrian Foreign Ministry in Damascus said the US is resorting to "cheap tactics" and fabrications to justify Obama's decision to arm the rebels.

In an interview for the Guardian, Mr. Cameron said "We do (share the US judgement). I discussed this with President Obama on my recent visit. Our intelligence agencies have been sharing information. We share their view that, as we put it, growing levels of information about chemical weapons used by the regime and no firm evidence that chemical weapons have been used by the opposition. I welcome this candid assessment by the Americans".

"I think it, rightly, puts back centre-stage the question, the very difficult question to answer but nonetheless one we have got to address: what are we going to do about the fact that in our world today there is a dictatorial and brutal leader who is using chemical weapons under our noses against his own people?" he added.

'Strong international response'

UK Foreign Secretary William Hague said the crisis in Syria demands a "strong, determined and co-ordinated response from the international community".

US officials confirmed that the president has authorised sending weapons to opposition forces after the White House disclosed that the Assad regime had crossed "clear red lines" by deploying chemical weapons, including the nerve agent sarin.

The UK led calls for the European Union to lift its arms embargo on Syria, but the British Government has stressed that no decision to provide weapons to rebels has been taken.

Mr. Hague said the British government would urgently discuss the international response to the US assessment of chemical weapons use.