Media playback is unsupported on your device Media caption UK Foreign Secretary William Hague: "Opposition groups must be committed to a democratic future"

Foreign Secretary William Hague has said he believes Syrian opposition groups could be a "credible, political alternative" to the Assad government.

Before meeting opposition leaders in London, Mr Hague said the UK wanted to recognise them as the legitimate voice of the people at "an early stage".

But he said he must be sure they had "genuine support" throughout Syria and were committed to a democratic future.

Up to 30,000 people have been killed in the 18-month conflict, the UK believes.

At Friday's meeting, Britain will urge the newly formed coalition to put forward a clear plan for political transition in Syria.

In return, the UK government is considering official recognition of the group as the legitimate opposition to President Bashar al-Assad's rule.

The UK could also push for the lifting of an EU arms embargo to allow the supply of weapons to rebel forces.

'Assurances needed'

Those trying to bring down President Assad's regime moved closer to a united front when the rival leaders of Syria's rebels formed a coalition - the National Coalition for Syrian Revolutionary and Opposition Forces - after months of bitter division.

OPPOSITION LEADERS Ahmed Moaz al-Khatib , pictured, is a former imam of the Umayyad Mosque in Damascus and a respected figure within Syria

, pictured, is a former imam of the Umayyad Mosque in Damascus and a respected figure within Syria He was imprisoned several times for criticising the Assad government before fleeing the country, settling in Cairo

He is not allied to any political party and is known as a moderate who has called for political pluralism and opposed sectarian divisions

Riad Seif is a veteran dissident who was once a prominent industrialist, owning large textile factories supplying firms such as Adidas

is a veteran dissident who was once a prominent industrialist, owning large textile factories supplying firms such as Adidas Since entering politics in 1994, he has called for Syria's economy to be opened up to new investment and for a crackdown on corruption

He has been detained on several occasions between 2001 and 2011

Suhair al-Atassi is a descendent of a famous political family and a woman who held one of the last open political discussion groups in Damascus. Profile: Ahmed Moaz al-Khatib Profile: Riad Seif

Its leaders, headed by President Ahmed Moaz al-Khatib and Vice-Presidents Riad Seif and Suhair al-Atassi, are holding talks with Mr Hague at the Foreign Office in London.

BBC diplomatic correspondent James Robbins says the UK wants to size them up and test their commitment to human rights and an inclusive government - in the event that they topple President Assad.

Ahead of the meeting, Mr Hague said he was "encouraged" by their commitment to respect human rights, promote political inclusiveness and uphold international law.

He said he would a statement to Parliament next week on the issue of whether the UK would recognise them as the sole legitimate representative of the Syrian people - but said he was keen to do this at an "early stage".

"I wanted to meet them myself before the UK takes that step," he said.

"We need their assurances about being inclusive of all communities, we need to see they have genuine support throughout Syria if we are to take that important step of recognition.

"We should do so in full possession of the facts and on the basis of discussions with them. The meeting is an important component of that and we will continue to work on this over the next few days."

France has already officially recognised the new coalition as in effect Syria's government-in-waiting.

But the UK and US have stopped short of doing this amid concerns about the background of some opposition elements and their involvement in violence and repression during the current conflict.

Some factions have also been linked to Islamic militant and terrorist organisations.

All options

We cannot stand still and just say we will leave things as they are William Hague, Foreign Secretary

Mr Hague said the situation in Syria - discussed by the UK's National Security Council on Thursday - was "grave and deteriorating".

Although no options had been ruled out, he said the UK's priority was still securing a negotiated solution as part of a process of political transition while continuing to provide humanitarian assistance.

He added: "A military victory of one side over the other would be a long, expensive process in terms of human life. Our top priority remains achieving a diplomatic and political solution...

"We cannot stand still and just say we will leave things as they are...but how we respond had to be well-judged and well-thought through."

Other options reportedly considered include arming groups opposed to the Assad government and creating UN "safe zones" inside Syria to protect civilians.

Mr Hague will meet his French counterpart at a meeting of EU foreign ministers in Brussels on Monday.

Opposition and human rights activists estimate that more than 36,000 people have been killed since the uprising against President Assad began in March 2011.