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David Cameron has said Britain has taken "no decision" to arm the Syrian rebels after the US declared it would provide them with military support.

But the prime minister backed the US assessment that the Syrian regime had used chemical weapons and described President Assad as a "brutal dictator".

Mr Cameron and leaders including President Obama discussed the conflict by phone ahead of next week's G8 talks.

Syria called claims it had used chemical weapons "a caravan of lies".

Russia says it is not convinced by the evidence presented by the US, and the UN General Secretary Ban Ki-Moon has said providing arms to either side would not help as there must be a political, not military, solution.

The White House said President Obama had made the decision to provide arms to the opposition after concluding the Assad regime was using chemical weapons.

It has not yet given details about what military aid might be provided but there has been speculation it could include small arms and ammunition, or possibly anti-tank weapons.

On the ground in Syria, there were reports of the fiercest fighting in months in the country's largest city, Aleppo. The UN said on Thursday that two years of conflict had killed at least 93,000 people.

On Friday evening, Mr Cameron held an hour-long video conference to discuss G8 summit priorities with President Obama, Germany's Chancellor Merkel, President Hollande of France and Italy's Prime Minister Letta.

A Downing Street spokesman said: "They discussed the situation in Syria and how G8 countries should all agree to work together on a political transition to end the conflict."

The G8 summit - a meeting of leaders of the world's foremost economic nations - takes place at Lough Erne, County Fermanagh, on Monday and Tuesday next week.

Mr Cameron backed the lifting of an EU arms embargo on Syria in May, but says he has made no decision to send arms to the Syrian opposition.

He is under pressure from some of his own MPs and the Labour Party to hold a vote in the Commons before any such decision is taken.

At a news conference in the Downing Street garden on Friday afternoon, when he also unveiled an economic package for Northern Ireland, Mr Cameron said samples from inside Syria showing evidence of chemical weapons had been tested by the UK Defence Science and Technology Laboratory at Porton Down.

There was "credible evidence of multiple attacks using chemical weapons", he added.

The UK believed that the weapons had been ordered by the Assad regime, Mr Cameron went on, but he said there was no credible reporting of chemical weapons being used by the opposition, although elements affiliated with al-Qaeda had attempted to acquire some for "probable use" in Syria.

Reservations about arming the rebels have been expressed by some Conservative MPs and, reportedly, some of Mr Cameron's own cabinet ministers.

Opponents say there is no way to guarantee that weapons will not fall into the hands of extremists within the Syrian opposition forces and argue that it could escalate the conflict further.

Mr Cameron said: "We have made no decision to arm the opposition but it was right to lift the arms embargo."

He added: "We will continue to support, train and assist and work with the opposition.

"Of course there are concerns about some of the opposition, but my argument is this: If we don't engage with elements of the opposition and encourage those that do have a positive pluralistic and democratic view about the future of Syria, we won't be able to influence the shape of that opposition."

He said there was a "brutal dictator who is using chemical weapons under our nose" and it was important to work with "our allies and friends in the region" to do everything possible to end the conflict.

"That is what we will do in the days and the weeks ahead," he said.

Labour leader Ed Miliband has argued that the government has been putting all its efforts into lifting the EU arms embargo - not on securing the Geneva II peace talks, which have already been postponed until at least next month.

In an article for Saturday's Independent, Mr Miliband writes: "An escalating death toll and this week's announcements from Washington show just how grave are the dangers confronting Syria.

"It is vital that at this G8 meeting every effort is made by David Cameron and other leaders to engage the Russians directly and enlist their support in bringing all sides to the negotiating table.

"The joint US-Russian initiative remains the only credible plan to secure a transition to an inclusive and sustainable peace settlement for Syria."