The EU has drawn up a "concept of operations" for the deployment of military forces in Libya, but needs UN approval for what would be the riskiest and most controversial mission undertaken by Brussels.

The armed forces, numbering no more than 1,000, would be deployed to secure the delivery of aid supplies, would not be engaged in a combat role but would be authorised to fight if they or their humanitarian wards were threatened. "It would be to secure sea and land corridors inside the country," said an EU official.

The decision to prepare the mission, dubbed Eufor Libya, was taken by the 27 governments at the beginning of April. In recent days, diplomats from the member states have signed a 61-page document on the concept of operations, which rehearses various scenarios for the mission in and around Libya, such as securing port areas, aid delivery corridors, loading and unloading ships, providing naval escorts, and discussing the military assets that would be required.

The planning has taken place inside the office of Catherine Ashton, the EU's foreign and security policy chief. Officials are working on an "A-plan", the operational instructions that would specify the size of the force, its equipment and makeup, and the rules of engagement.

Diplomats and officials said this would not be finalised unless a request for an EU military mission came from the UN body the Office for the Co-ordination of Humanitarian Affairs (Ocha).

Valerie Amos, the head of Ocha, has privately told EU leaders she is reluctant to make the request and wants to explore all civilian options for the aid operation before seeking military help.

Amos said on Monday the Libyan government has promised the UN access to the besieged rebel city of Misrata, although they had not guaranteed a ceasefire during such a mission. She was in talks with the Libyan prime minister in Tripoli on Sunday and said she would send a team to the city of 300,000 as quickly as possible, adding that she was "deeply concerned" about the safety of civilians.

The EU has established an operations headquarters in Rome under the command of an Italian rear-admiral as part of its plan for a military deployment to Libya. Ashton has written to Ban Ki Moon, the UN secretary-general, offering the military assets, but the overture has been declined.

"The operation is agreed. It's ready to go when we get the nod from the UN," said the EU official. But behind the scenes in Brussels, there is much ambivalence as well as attempts at point-scoring between the bigger member states.

Diplomats say Ashton is pushing for a UN consent under strong pressure from the French, which is generally keen to promote projects supporting European defence and security policy.

With the situation in Libya, particularly Misrata, getting more critical, diplomats in Brussels say the pressure is mounting on the UN to authorise the EU force. "We're at the point of saying we may need to support aid being delivered," said one EU diplomat in Brussels. "So you need people with military capability. The EU has two battle groups ready."

Under a policy going back several years but seldom used, Brussels has a roster of battle groups, with two on permanent standby, comprising a force of about 1,500. The main battle group that would be deployed is German-dominated, which could trigger a row.

France and Britain have been the main hawks on Libya while Germany has been the most vocal opponent of the bombing campaign. Berlin stunned its allies by abandoning the UK, France, and the US in the security council vote last month that mandated the bombing.

Berlin has since promised it would commit forces for a humanitarian mission, but Paris and London would be reluctant to let Germany take the lead, fearing it would be overcautious and restrict the mission's scope.

The Nato alliance is keen to let the EU take over any armed escorts for an aid mission, seeing little role for itself. "I would appreciate it if the EU could take the initiative on the delivery of humanitarian aid," the Nato secretary-general, Anders Fogh Rasmussen, said last week. "Nato has no intention to play a leading role."