By David Brunnstrom
BRUSSELS (Reuters) - NATO responded on Tuesday to a Congressional threat to cut off funding to the U.S. military involvement in Libya by saying the United States was providing unique assets essential for the success of the mission.
U.S. lawmakers have questioned the legality of continued use of the U.S. military in Libya and House of Representatives Speaker John Boehner said last week Congress could cut funding for the mission.
NATO spokeswoman Oana Lungescu said debates were normal in democracies and the alliance would not interfere in these.
"But what I can say is that we are very grateful for the assets that the U.S. is providing and those are unique and essential assets to the success of this mission," she told a NATO news briefing.
"We have a very clear United Nations mandate ... We are fulfilling that mandate. We have the commitment to see this through because this is what the international community wants us to do and this is what the people of Libya need."
While the United States has stepped back from a leading role in the strike mission NATO took over on March 31, it has continued to provide essential assets, including reconnaissance planes, air-to-air refueling planes and armed drones.
However, U.S. law prohibits U.S. armed forces from being involved in military actions for more than 60 days without congressional authorization, with a 30-day pullout period. Boehner says these 90 days are up on Sunday.
Criticism of the conflict has been fueled by unease in both U.S. political parties over a third war on top of Afghanistan and Iraq and worries about more costs in a time of massive debt.
Defense Secretary Robert Gates urged Congress on Sunday not to cut off funding for the mission and predicted that the operation against Muammar Gaddafi's forces would "end OK."
The debate in Washington comes at a time when European participants in the bombing mission have questioned their ability to sustain it long-term, and NATO allies not currently taking part have been reluctant to offer support.
On Tuesday, British Prime Minister David Cameron said Britain could keep up its campaign in Libya as long as necessary, despite warnings by military chiefs that their forces are overstretched.
The Daily Telegraph quoted a briefing paper from Air Chief Marshall Simon Bryant, head of Royal Air Force combat operations, as saying its ability to deal with unforeseen events would be eroded if the campaign lasted beyond September.
France has indicated the mission's only aircraft carrier, the Charles de Gaulle will have to be withdrawn in the autumn. The rationale of the mission has been questioned in Italy.
Lungescu said NATO defense ministers had committed this month to sustain the mission as long as it took, while NATO military spokesman Wing Commander Mike Bracken said the military command would work with whatever assets were available.
"If some nations at one stage need to withdraw their support in assets, other nations have been forthright in providing additional support in other areas," he said.
Frustration has grown among allies at the failure of more than three months of bombing to dislodge Gaddafi and debate has been fueled by a series of negative incidents in recent days.
NATO has admitted it had struck a rebel military column last week and says a malfunctioning bomb appeared to blame for killing civilians in Tripoli on Sunday. On Tuesday, it had its first loss of the air campaign -- an unmanned U.S. helicopter.
Italian Foreign Minister Franco Frattini said on Monday that civilian deaths risked NATO's credibility in a mission supposed to protect civilians.
(Editing by Angus MacSwan)