Citing the War Powers Resolution, which requires presidents to notify Congress within 48 hours of beginning military operations, the president said in a letter to House and Senate leaders that U.S. forces "commenced operations to assist an international effort authorized by the United Nations Security Council and undertaken with the support of European allies and Arab partners, to prevent a humanitarian catastrophe and address the threat posed to international peace and security by the crisis in Libya."
He said the strikes against Libya's air defense systems and military airfields to establish the no-fly zone were authorized under U.N. Security Council Resolution 1973.
"These strikes will be limited in their nature, duration, and scope," he wrote. "Their purpose is to support an international coalition as it takes all necessary measures to enforce the terms of U.N. Security Council Resolution 1973. These limited U.S. actions will set the stage for further action by other coalition partners."
The president said Gadhafi had persisted in attacking Libyan civilians and failed to implement a cease-fire announced last week by his government.
"(Gadhafi's) continued attacks and threats against civilians and civilian populated areas are of grave concern to neighboring Arab nations and ... constitute a threat to the region and to international peace and security," Obama said.
The no-fly zone over Libya will stretch 621 miles from the rebel stronghold of Benghazi to Tripoli, U.S. Army Col. Carter Ham, head of the U.S. Africa Command, said Monday.
"Our actions today are focused on extending the no-fly zone southward, then westward from Benghazi," Ham said during a military briefing.
He said he anticipated the no-fly zone will "soon extend to Brega, Misurata and then to Tripoli."
Coalition forces from the United States, Belgium, Britain, Canada, Denmark, France, Italy, Norway, Qatar and Spain are participating in air and sea attacks on troops loyal to Gadhafi.