NATO defence ministers agreed today to send more naval ships off Libya's coast and to renew military planning for a possible no-fly zone pending a UN mandate, the alliance's chief said.

"It has been decided to increase the presence of NATO maritime assets in the central Mediterranean," NATO Secretary General Anders Fogh Rasmussen told a news conference after a meeting of defence chiefs.

The naval ships would bolster surveillance of Libya and monitor an existing arms embargo against the country, where Moamer Kadhafi's troops are battling opposition forces.

The ministers also called for urgent "detailed planning" for humanitarian efforts and for possible measures to enforce the arms embargo in case required by any new UN Security Council resolution, he said.

There was no decision to launch a no-fly zone against the regime -- a step favoured by Britain and France -- but the alliance endorsed further military planning if the United Nations endorsed the move.

"We considered as well initial options regarding a possible no fly-zone in case NATO were to receive a clear United Nations mandate," he said.

"Ministers agreed further planning will be required."

Despite signs of disagreement over how to respond to the crisis, the NATO chief said the ministers were "united".

"Our message today is NATO is united, NATO is vigilant, NATO is ready to act," he said.

Rasmussen did not say how many ships would be deployed towards Libya in addition to vessels already deployed by alliance states.

The United States has already sent three warships to the area with hundreds of Marines on board, including the amphibious assault vessel the USS Kearsarge.