Concern over the legality of the military action in Libya reignited on Monday as rebel forces surged into the space created by the international bombardment of Colonel Gaddafi's military.

Philippe Sands QC, professor of international law at University College London, warned that coalition forces were facing a "major problem" to justify their latest strikes on legal grounds and Lord Ashdown, the former high representative for Bosnia and Herzegovina, said the coalition forces led by Britain, France and the US were facing "a moment of danger" over the legality of their actions. He said "continued support for this looks as though it is leading to support for regime change, which legally is beyond the [United Nations] security council resolution".

Legal experts said the international coalition may have overstepped what was agreed by the UN resolution sanctioning military action to "take all necessary measures … to protect civilians and civilian populated areas under threat of attack".

Professor Nicholas Grief, director of legal studies at the University of Kent, said it was possible there could be an attempt to bring the matter before the international court of justice. Others said the coalition forces were within the bounds of legality and could continue to attack Gaddafi's military positions as long as they posed any future threat to civilian populations.

Concern grew as Sergei Lavrov, Russia's foreign minister, said he believed the military action was in breach of international law. "We consider that intervention by the coalition in what is essentially an internal civil war is not sanctioned by the UN security council resolution," he said. Russia abstained from the vote which resulted in resolution 1973.

Britain said the strikes remained legal. "The UN resolution's point of ensuring that civilians could be protected allows the international coalition to take action against those who are threatening civilians," said Alistair Burt, Foreign Office minister. "The Gaddafi forces have been threatening civilians through the advance of their military machine. In order for that threat to be lifted, action has been taken as we have seen. It is very important for us and for everyone that what has been done is under the terms of the UN resolution."

But Sands said it was becoming increasingly hard to justify the strikes on the Libyan leader's forces as pre-emptive.

"The resolution is concerned with the protection of civilians, so a military attack on Gaddafi's retreating forces could only be justified if it could be shown to be related to that objective," he said.

"It is difficult in international law to argue for a pre-emptive use of force to protect civilians from a possible threat that might arise in the future. We don't know if there is evidence to show that a failure to attack Ghadaffi's forces would lead to a regrouping that would lead in turn to attacks on civilians. Pre-emption is a major problem because it is seen as a slippery slope, and rightly so."

His concern was echoed by Grief, who said the latest strikes provided evidence the coalition had taken sides and "may have gone beyond the terms of the resolution" which he said must be interpreted narrowly, under international law. "It is almost as if we have entered the fray openly on behalf of the rebels. We have taken sides, Paradoxically, there is a UN arms embargo in place but raiding Sirte is even better for the rebels than if we were arming them." he said.

He said the most recent actions seemed different from the initial attacks around Benghazi, where a stronger argument could be made that action was immediately necessary to protect civilians in support of the resolution.

"I am not sure the latest strikes are in the same category. That is why I am concerned we are going beyond the terms of the security council resolution."

He said the only way the coalition could be restrained in the immediate future was if the security council adopted a fresh resolution, but he said that was very unlikely given the permanent membership of the council by the leading members of the military coalition. "There could be an attempt to bring the matter before the international court of justice, but it is very difficult to see that happening."

But Malcolm Shaw QC, senior fellow at Cambridge University's Lauterpacht centre for international law, argued the coalition forces were still operating within the bounds of legality.

"We are into elastic now, and how far can you stretch the resolution?" he said. "Where you have concentrations of Libyan troops who still pose a credible threat to civilians or civilian population areas then I think there is still cover for action against military objectives," he said. "It would be difficult to say that if there are tanks outside Sirte that they are no longer a threat to places further up the road at Ajdabiya or Benghazi. If the rebels were hit and scattered then Gaddafi's tanks could go in.

"When the Russians say you can't intervene in civil war to assist the rebel side they are right, because that is international law . But the security council resolution trumps that. The resolution does not say protect civilians from attack, but protect them from the threat of attack, so as long as the Libyan government maintains a fighting force and is maintaining a forcible stance then those forces are legitimate targets. The authorisation to use force is clear-cut. The question is how far you go. The answer for me is far down the line."

Who is in charge?

Who is running the military operation?

Officially Nato has taken over all military operations. But at the moment it is only running the naval blockade, which is enforcing the arms embargo, and the no-fly zone. The Nato commander, Lieutenant General Charles Bouchard, will assume control of the most controversial operations, the air strikes against Gaddafi's ground forces, in a few days' time.

Why is there a delay in Nato taking over air strikes?

Bouchard said that the handover would take some days because it was "complex". The delay gives more time to the existing coalition, co-ordinated by the US but led by the French and British, to continue to choose their own targets for bombing.

Will the handover to Nato control make any difference?

Nato's 28-member countries agreed rules of engagement for air strikes on Sunday. As sceptics like Turkey and Germany were involved in the draft, the rules are likely to be more restrictive that those being used by the French and the British, who have been bombing Libyan government forces across the country. Julian Borger