British military officers will be sent to Libya to advise rebels fighting Colonel Muammar Gaddafi's forces, the UK government has said.
Foreign Secretary William Hague said the group would be deployed to the opposition stronghold of Benghazi.
The BBC understands 10 UK officers and a similar number from France will give logistics and intelligence training.
Mr Hague said it was compatible with the UN resolution on Libya, which ruled out foreign military ground action.
He stressed that the officers would not be involved in any fighting and the move was needed to help protect civilians.
Support and adviceThe UN Security Council resolution, passed in March, authorised a no-fly zone over Libya.
Mr Hague said: "The [UK] National Security Council has decided that we will now move quickly to expand the team already in Benghazi to include an additional military liaison advisory team. This contingent will be drawn from experienced British military officers.
"These additional personnel will enable the UK to build on the work already being undertaken to support and advise the NTC [opposition National Transitional Council] on how to better protect civilians.
Analysis
William Hague insists this limited deployment does not mean "boots on the ground". The British military officers will not be in uniform and will not join the battle.
We are told they'll be providing "non-lethal assistance". But those carefully chosen words can't hide their real purpose - that is to make the rebels a more effective fighting force.
The rebels will be trained in the communications, logistics and intelligence skills used by a modern military. It's hard to see this intervention as purely a humanitarian response, though the government insists the assistance falls within the mandate of the UN Security Council.
It will still be interpreted as "mission creep". It will also prompt more questions as to what could follow.
The foreign secretary says there are no plans to send in British combat troops to Libya - what he interprets as real boots on the ground. But he hasn't ruled out arming the rebels.
"In particular they will advise the NTC on how to improve their military organisational structures, communications and logistics, including how best to distribute humanitarian aid and deliver medical assistance.
"In doing so, we will coordinate closely with other international partners also assisting the NTC."
The officers will be wearing civilian clothing, not uniforms, but are likely to carry sidearms.
The announcement comes as hundreds of people are feared dead in the bombardment of the town of Misrata by pro-Gaddafi forces.
The UK has already supplied body armour and telecommunications equipment to help the rebels.
On Monday, International Development Secretary Andrew Mitchell said the UK would provide £2m to help civilians flee Misrata by boat.
Mr Hague said: "We have stepped up our contribution to international efforts to relieve the humanitarian crisis in Libya, in particular in Misrata, and a UK diplomatic team led by Christopher Prentice has been liaising closely with the opposition in Benghazi."
French Foreign Minister Alain Juppe has said France is opposed to the idea of sending coalition ground troops into Libya, even special forces to guide air strikes, to break the military stalemate.
'Mission creep'Former Liberal Democrat foreign affairs spokesman Sir Menzies Campbell warned against becoming bogged down in Libya, in similar fashion to what happened to the US in Vietnam.
He said: "Sending advisers for a limited purpose is probably within the terms of [United Nations] Resolution 1973, but it must not be seen as a first instalment of further military deployment.
"Vietnam began with an American president sending military advisers. We must proceed with caution."
Labour MP David Winnick, who backed last week's demands to recall Parliament from the Easter recess so MPs could debate the Libya situation, criticised the deployment of British officers.
He said: "However much one despises the brutality of the Gaddafi clan which rules Libya, the fact remains that there is a danger of mission creep.
"There is a civil war in Libya and this is a big escalation of Britain's involvement. I don't think there is an appetite in Britain for military intervention.
"Having been engaged in two wars in nine years in Muslim countries, it would be unwise to become involved in a third."
'Precision firepower'Shadow Foreign Secretary Douglas Alexander said Labour supported the enforcement of the UN resolution, which excluded an occupying force.
"That resolution allows for the international community to take action such as that announced today to support the protection of civilians and help ensure their humanitarian needs are met, which is the basis for the government's decision," he said.
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Meanwhile, British military chiefs have been giving details of the latest action by the RAF to enforce the no-fly zone.
Maj Gen John Lorimer said Tornado and Typhoon aircraft attacked rocket launcher vehicles and light artillery observed firing on Misrata, and a second pair of RAF planes destroyed a gun and tank on a transporter.
Tomahawk cruise missiles were also fired by the nuclear-powered submarine HMS Triumph.
Maj Gen Lorimer said: "It will take time for the full impact of these attacks to become clear.
"But they do illustrate in the clearest manner Nato's resolution to take all necessary action to safeguard wherever possible the Libyan people under threat of attack and its ability to strike, with sophisticated targeting and effective precision firepower, at the heart of the apparatus used by Colonel Gaddafi to terrorise the civilian population."
Fluid situationNato has been giving an assessment of how the no-fly zone over Libya has been progressing, three weeks into the operation.
Brig Gen Mark Van Uhm, chief of Allied operations, said almost 2,800 sorties had been flown over Libya, destroying over a third of Col Gaddafi's military assets.
He said the situation on the ground remained fluid and was changing daily.
The World Food Programme said it had started moving supplies through a new humanitarian corridor into western Libya, with the first convoy crossing from Tunisia on Monday.
It was loaded with enough wheat flour and high-energy biscuits to feed 50,000 people for 30 days.
The food will be delivered to several Libyan cities, although not Misrata which is said to be critically short of supplies.
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