WASHINGTON (AFP) – President Barack Obama insisted that the current US military action in Libya was legal, rejecting rising criticism from Congress over the goals and justification of the operation.

In a 30-page report to lawmakers, the White House argued that US participation in the NATO-led assault on Moamer Kadhafi's forces did not require congressional authorization as the US role was only a supporting one.

The report said the US involvement in the UN-authorized operation did not rise to the kind of direct, offensive warfare that needed to be endorsed by lawmakers under the 1973 War Powers Resolution.

"US forces are playing a constrained and supporting role in a multinational coalition," the report said, noting that the use of force was being used solely to protect civilians, enforce a no-fly zone and an arms embargo.

"US operations do not involve sustained fighting or active exchanges of fire with hostile forces, nor do they involve the presence of US ground troops, US casualties or a serious threat thereof, or any significant chance of escalation into a conflict characterized by those factors," the report said.

A senior administration official added: "we are not engaged in any of the activities that typically over the years in war powers analysis has been considered to constitute hostilities within the meaning of the statute."

The report also put US costs at $715 million for military and humanitarian operations in Libya since unrest began in the north African country earlier this year and the projected price tag through September is about $1.1 billion.

The document was compiled after House Speaker John Boehner sent a scathing letter to the president warning that US operations would be illegal come Sunday because they lacked formal congressional approval.

The War Powers Resolution gives presidents 60 days to get authorization for a military deployment and, failing that, sets a further 30 days to withdraw them from harm's way.

Republican leadership aides said that they were studying the report, which raised "creative arguments" that needed to be further explored.

"Regardless, the commander-in-chief has a responsibility to articulate how US military action is vital to our national security and consistent with American policy goals," said Boehner spokesman Brendan Buck.

"With Libya, the President has fallen short on this obligation."

The administration said that the while it did not believe that it required formal authorization for the Libya operation, it would welcome a statement of support for the mission.

White House spokesman Jay Carney also said that the administration had initiated more than 40 points of contact with Congress, disputing claims that lawmakers had not been sufficiently kept up to speed.

Political maneuvering in Washington over Libya took place as Western officials insisted their intervention was working and could be sustained, and as rebels made advances on the road to Tripoli.

But after 10 weeks of air strikes against Kadhafi's forces and defections from his regime, it remained unclear how long the Arab strongman could last out, and whether the NATO-led mission would dislodge him.

In another sign of angst in the Capitol over the mission, anti-war Democratic lawmaker Dennis Kucinich and a bipartisan group of nine other lawmakers filed suit alleging Obama bypassed Congress in ordering the mission.

"Neither NATO nor the UN trump the Constitution of the United States," Kucinich said, adding later on CNN: "If it looks like a war, it's a war."

Washington took a prime forward role in Libya after the UN Security Council passed a resolution on March 19 allowing for air strikes against Libyan regime forces in order to protect civilians.

But aides said that Obama was good to his word and pulled US forces back into a support mission after an initial blitz of airstrikes and the operation is now under NATO command with Britain and France the most active members.

But another senior US official told reporters on Wednesday that the mission was going well, and had yielded important successes, and argued that Obama's decision to launch military action had prevented a feared civilian massacre.

"The bottom line is that lives have been saved; Kadhafi's advances have been stopped, the opposition and the Libyan people have had time and space to organize," the official said.

"Right now we see a situation in which time very much is working against Kadhafi."