Britain is to break new ground in its response to the crisis in Syria by opening talks with armed opposition groups, as ministers acknowledge that militants are increasingly setting the agenda.

The foreign secretary, William Hague, will announce the move at a conference in Doha. David Cameron will give his blessing to the move, which is to be announced by William Hague at a conference in Doha, when he visits Syrian refugees at a camp in Jordan on Wednesday, on the final day of his Middle East and Gulf tour. More than 360,000 Syrian refugees are registered in neighbouring countries, 110,000 in Jordan.

The prime minister earlier signalled Britain's growing concerns at the "appalling slaughter" in Syria when he declared he would be prepared to allow President Bashar al-Assad to be given safe passage to a third country.

The move was criticised by human rights groups, who warned that it might increase the violence. Kenneth Roth, the director of Human Rights Watch, tweeted: "PM Cameron, be careful: offering immunity gives Assad licence to kill as many as needed, leaving only if he loses power."

Cameron, who will announce an extra £14m in humanitarian aid for Syrian refugees, has sanctioned a change of tack in contacts with opposition groups amid fears a prolonged conflict will fuel extremism. "The longer this goes on, the more that it can promote and drive extremism and we'll see instability in the region as well," Cameron told al-Arabiya television in an interview released as he arrived in Saudi Arabia for a short visit on Tuesday.

Officials who are talking to opposition groups, led by Britain's special representative John Wilks, are not in contact with extremists who are seeking to exploit the conflict in Syria. Wilks is only talking to groups that would play a key role in the transition to a new government before joining a new administration in Damascus. One source said: "This is not a ratcheting-up and it is certainly not a precursor to arming anyone. This is reinforcing an existing dialogue."

Cameron has authorised discussions with militants after Wilks advised there was fluidity between opposition leaders involved in political development and those involved in military planning. The source said: "They tend not to have the distinction we have between the prime minister and the chief of the defence staff."

Hague will make clear, when he announces the contacts at a conference in Doha hosted by the Qatari government, that Britain will tell the armed opposition groups they must respect human rights and co-operate with aid agencies to improve access for humanitarian aid.

Officials are playing down parallels with the Libyan conflict, when Britain provided "non-lethal" military equipment, such as communications equipment, to the opposition groups. "We effectively provided the air force," one source said.

Help for the Libyan opposition was authorised by Dominic Grieve, the attorney general, using the UN security council resolution. This permitted military force to protect the civilian population from Muammar Gaddafi's forces. It is understood Grieve has not had to authorise the contacts with armed Syrian groups. No security council resolution has been agreed on Syria because Russia and China oppose any such move.

Cameron – who will announce that a third of the extra £14m in aid will be spent feeding 23,000 Syrian refugees in Iraq, Jordan, Lebanon and Turkey – made clear that Britain needs to intensify its response to the crisis in Syria. He told al-Arabiya: "We must ask ourselves what more can we do? How can we help the opposition?"

Asked whether Britain would help arm the rebels, he said: "We are not currently planning to do that. We are a government under international law and we obey the law."

More than 30,000 people have been killed in Syria since the conflict started 19 months ago. More than a million have been displaced internally and 2.5 million Syrians are in need of humanitarian assistance.

The extra £14m will take Britain's contribution to the Syrian crisis to £53.5m. This makes Britain the second largest national donor after the US.

Most of the funding (£9.5m) will be used to provide blankets for 30,000 families in Syria as the cold weather closes in at night. It will also pay for food for 71,000 people and help for 8,300 Palestinian refugees in Syria.

The prime minister's remarks were released as he arrived in Jeddah for a meeting with King Abdullah. Saudi Arabia is a strong supporter of the Syrian opposition.

The Foreign Office played down his pronouncement that Britain might agree to allowing Assad safe passage to a third country. It said: "We have been clear that Assad should face justice and that it is for the Syrian people, including the opposition, to decide the details of a transition including the options for Assad. The longer the killing goes on, the fewer options Assad will have."