Britain is to review the EU arms embargo on Syria as part of a wholesale change in strategy in the wake of Barack Obama's re-election that could lead to the eventual arming of the rebel forces fighting to overthrow Bashar al-Assad.
As David Cameron said he would press Obama to make Syria a priority, No 10 officials indicated that the prime minister now wants to put every possible measure to remove Assad "back on the table".
Cameron's visit to the Zaatari Syrian refugee camp in Jordan on Wednesday, in which he heard "appalling stories" of suffering, persuaded him that Britain and its allies need to review their strategy, a source said. Britain's national security council will discuss the crisis in special session next week.
It will include a review of the EU ban on providing weapons to all sides in Syria. Officials say that the embargo includes the principle of "proportionality" which suggests the restriction could be relaxed in the event of a humanitarian disaster.
Evidence of a British rethink on the crisis came on a day when rebels fired mortars at a presidential palace in Damascus and as different elements of the divided Syrian opposition met in the Qatari capital Doha to try to close ranks and form a transitional government for the post-Assad era.
In another indication of regional tensions, Turkey confirmed that it is to make an official request to Nato to station Patriot missiles along its border with Syria. The move follows several incidents of shelling across the 560-mile frontier. It could also be linked to the ideas of establishing a safe zone or no-fly zone in the border area.
The moves by Britain and Turkey both seemed to anticipate a bolder approach from Obama to end the conflict that has claimed an estimated 35,000 lives since the bloodiest of the Arab spring uprisings erupted in March 2011. On average 100 to 150 people now die every day.
The text of the EU embargo, agreed two months after the conflict began, says: "By way of derogation … the competent authorities in the member states … may authorise the sale, supply, transfer or export of equipment which might be used for internal repression, under such conditions as they deem appropriate, if they determine that such equipment is intended solely for humanitarian or protective use."
Cameron made clear he believes that stage may have been reached after he visited the refugee camp, where 110,000 Syrians are sheltering. "I think what I have seen and heard today is truly appalling," said. "I think [with] a re-elected president [Obama] with a new mandate … it's really important to discuss what more we can do to help resolve the situation."
Underlining the shift, the foreign office announced on Wednesday that it will talk to "military figures in the armed opposition" though it insists it has no plans to arm the rebels – the suspicion of those who fear a rerun of Nato's intervention in Libya last year.
Previously the Foreign Office had sanctioned contact only with "political representatives of armed Syrian opposition groups".
William Hague, the foreign secretary, said in a statement to MPs that Britain would "adhere to our clearly stated policy of only supplying non-lethal support to the unarmed opposition".
But No 10 believes there is a mismatch in which the EU and the US provide only "non-lethal" help to the rebels while Russia and Iran provide resources and weapons to Syrian government forces.
It is understood that Britain may review the EU embargo as a tactical ploy to persuade the Russians and Chinese, who have pledged to veto any UN security council resolution, to change position.
The prime minister wants Britain's security council to examine the viability of creating safe havens, an idea championed by Turkey, and to assess whether Assad could be persuaded to stand down by the offer of safe passage to a third country.
Officials acknowledge that it would be difficult to secure such havens without imposing a no-fly zone over parts of Syria. This is deemed impossible because China and Russia would block such a move, which would be difficult to enforce in the face of Syria's powerful air force.
But Cameron said he was determined to act across a range of fronts. "That means more help for the opposition, more pressure at the UN, more help for the refugees, more work with the neighbours but also a general sort of: 'Look let's be frank what we've done for the last 18 months hasn't been enough.' The slaughter continues, the bloodshed is appalling, the bad effects it's having on the region, the radicalisation but also the humanitarian crisis that is engulfing Syria. So let's work together on really pushing what more we can do, what other steps we can take to hasten the end of this regime."
The No 10 source said: "Today is the moment the prime minister came and saw for himself what is happening. This is the moment to get some impetus going forward. We want to put everything on the table."
The main Syrian opposition group, the Syrian National Council, discussed electing a new leader and executive committee on Wednesday. It and other groups will meet on Thursday to form a new 50-member civilian group that will later choose a temporary government for Syria and, western governments hope, improve coordination with armed groups.