The 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.
The Prime Minister, who will speak to President Obama by telephone later today ahead of the G8 summit, said Bashar Assad's use of the weapons against his own people "under our noses" must be addressed.
In an interview for the Guardian, Mr Cameron said: "We do (share the US judgment). 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?"
Earlier the British foreign secretary William Hague has said Britain will be "urgently" discussing the international response to the US assessment of chemical weapons use as he said the government agrees with the US assessment that the Syrian regime had used chemical weapons.
Speaking ahead of the G8 summit, Mr Hague said the crisis demanded a "strong, determined and co-ordinated response from the international community".
It is believed that Britain, the United States and France are set to hold emergency talks to discuss how and when to arm the Syrian rebels and whether to impose a no-fly zone.
Mr Hague's comments came the day after it emerged that US President Barack Obama had approved arming the Syrian rebels for the first time in response to the use of chemical weapons.
Intelligence 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 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.
Speaking today William Hague said: "We agree with the US assessment that chemical weapons, including sarin, have been used in Syria by the Assad regime."
"The United Kingdom has presented evidence of the use of chemical weapons in Syria to the UN investigation, and we have been working with our allies to get more and better information about the situation on the ground. We condemn the deplorable failure of the Assad regime to co-operate with the investigative mission.
"As I said in Washington on Wednesday, the crisis demands a strong, determined and co-ordinated response from the international community.
"We have to be prepared to do more to save lives, to pressure the Assad regime to negotiate seriously, to prevent the growth of extremism and terrorism, and to stop the regime using chemical weapons against its people.
"We will be discussing that response urgently with the United States, France and other countries, including at the G8 this week."
The White House yesterday indicated that it would further increase the "scope and scale" of assistance to opposition forces, including direct "military support".
But there was no immediate announcement of what that would comprise or the timescale for accelerating support.
Deputy national security adviser Ben Rhodes said: "The United States and the international community have a number of other legal, financial, diplomatic, and military responses available.
"We are prepared for all contingencies, and we will make decisions on our own timeline."
Mr Rhodes said US intelligence services had "high confidence" that weapons including the nerve agent sarin had been used on a small scale on multiple occasions - causing at least 100-150 deaths.
There was "no reliable, corroborated reporting" to back claims that the rebels either possessed or had used them.
The UK and France have led a successful joint bid to lift an EU embargo on supplying arms to opposition forces although Prime Minister David Cameron stressed yesterday that no decision had been taken to arm the rebels.
He is due to hold talks with Russian president Vladimir Putin at Downing Street on Sunday as part of efforts to break the international deadlock.
The crisis will also be high on the agenda when they join other world leaders, including Mr Obama and French president Francois Hollande, at the G8 summit in Northern Ireland on Monday.
Walid Safur, British representative of the opposition Syrian National Coalition, said today the rebels need anti-aircraft and anti-tank missiles.
He provided assurances that any arms would not fall into the hands of extremist elements fighting with the opposition.
He told BBC Radio 4's Today programme: "We would like to see sophisticated weapons to bring down any aircraft and to deter this regime from bombarding the urban and civilian areas.
"Diplomacy has failed so far but however I would like to say that when weapons are given to the right hands, it will not go into the wrong hands."
Tory MP John Baron, a member of the Foreign Affairs Select Committee, said there were still questions over how to make sure extremists do not get any arms supplies.
He stressed the need for a diplomatic and political solution and urged the Prime Minister to allow a vote in Parliament on any British decision to send weapons.
"We in the UK do not have to follow the US. Good friends sometimes say to each other, look, you're making a mistake," he told Today.
"We have encouraged David Cameron to admit that this decision to arm the rebels or step up in any way UK support for the Syrian rebels will be subject to a debate and vote in Parliament."
Meanwhile, former Lib Dem leader Sir Menzies Campbell warned that the UK must not be a "copycat" of the United States.
He said: "The argument against the supply by the UK of arms to the Syrian rebels remains powerful and cogent.
"The same questions go unanswered. How would you prevent arms falling into the hands of jihadists, if not now, then in the future?
"What evidence is there that Assad will change his policies so long as he has the powerful support of Russia and the arms supplies that go with it?"
Additional reporting by PA