UPDATE 3-U.S. House Republican Leader Warns Obama over Libya

* Boehner says Obama could be violating law as of Sunday

* House Speaker demands answers by Friday

* White House says preparing information for Congress

(Adds White House comment in paragraphs 11-12)

By Susan Cornwell

WASHINGTON (Reuters) - House Speaker John Boehner warned President Barack Obama Tuesday that he was skating on thin legal ice by keeping U.S. forces involved in Libya for nearly three months without the authorization of Congress.

The letter from the Republican leader of the House of Representatives to the Democratic president threatened to turn lawmakers' unease over the Libyan conflict into a clash between Congress and the White House over constitutional powers.

Boehner accused Obama of "a refusal to acknowledge and respect the role of Congress" in military operations and a "lack of clarity" about why the U.S. was still involved in Libya.

He asked Obama to explain the legal grounds for the war by Friday, adding that by Sunday Obama would be in violation of the 1973 War Powers Resolution if nothing changed.

The U.S. Constitution says that Congress declares war, while the president is commander-in-chief of the armed forces. The War Powers Resolution sought to resolve the tensions in these roles and was passed by Congress over a veto by President Richard Nixon.

No sitting president has ever recognized the resolution, which prohibits U.S. armed forces from being involved in military actions for more than 60 days without congressional authorization, and includes a further 30-day withdrawal period. Boehner said the 90 days expires on Sunday.

"It would appear that in five days, the administration will be in violation of the War Powers Resolution unless it asks for and receives authorization from Congress or withdraws all U.S. troops and resources from the mission," Boehner said in the letter, which was released by his office.

"Have you ... conducted the legal analysis to justify your position?" he asked. "Given the gravity of the constitutional and statutory questions involved, I request your answer by Friday, June 17, 2011."

CONSULTATIONS

Obama notified Congress in March that the United States was taking part in a multinational operation conducting air strikes to protect Libyan civilians from by Muammar Gaddafi's forces. Obama did not ask for congressional authorization.

There are no U.S. troops on the ground in Libya, where NATO is leading the intervention with the United States providing logistical support and intelligence.

The White House says it has consulted regularly with lawmakers on the war and officials have suggested that the limited U.S. action might not meet the War Powers threshold.

White House spokesman Tommy Vietor said administration officials have testified at more than 10 hearings on Capitol Hill, which included substantial discussion of Libya. Officials also took part in more than 30 briefings with lawmakers and staff and would keep engaging with Congress, he said.

"We are in the final stages of preparing extensive information for the House and Senate that will address a whole host of issues about our ongoing efforts in Libya, including those raised in the House resolution as well as our legal analysis with regard to the War Powers Resolution," Vietorsaid.

If Obama did ask for congressional authorization, it is not clear he would get it. The Democratic-controlled Senate has not tried to pass a non-binding resolution supporting the war.

Two House lawmakers, Democrat Dennis Kucinich and Republican Walter Jones, said they would file a lawsuit in federal court concerning Obama and the Libyan war.

Earlier this month, a House majority passed a resolution accusing Obama of not having offered a "compelling rationale" for the Libyan war and demanding information about its costs and scope by Thursday June 16. Boehner's letter indicated that lawmakers are still waiting for answers.

That resolution noted that Congress has the authority to cut off funds for military operations, implying this might be considered if the Obama administration did not respond. (Additional reporting by Laura MacInnis; Editing by Cynthia Osterman and Christopher Wilson)