President Barack Obama formally informed Congress on Friday that he had sent Marine units to Libya and Yemen on an open-ended mission in the aftermath of attacks on American diplomatic missions there. He cited the bloody assault that claimed the lives of U.S. Ambassador to Libya Chris Stevens and three aides at the U.S. Consulate in Benghazi.

"Although these security forces are equipped for combat, these movements have been undertaken solely for the purpose of protecting American citizens and property," Obama said in a letter to Republican House Speaker John Boehner.

"These security forces will remain in Libya and in Yemen until the security situation becomes such that they are no longer needed," the president wrote.

"These actions have been directed consistent with my responsibility to protect U.S. citizens both at home and abroad, and in furtherance of U.S. national security and foreign policy interests, pursuant to my constitutional authority to conduct U.S. foreign relations and as Commander in Chief and Chief Executive," he continued. "I appreciate the support of the Congress in these actions."Obama wrote the letter in compliance with the 1973 War Powers Act that sought to curtail a president's ability to make war overseas without congressional consent. But he earlier had ignored congressional calls to get formal congressional authorization under that law for U.S. military involvement in the NATO-led campaign to oust Libyan strongman Moammar Gadhafi, saying it was not required because Americans were not in danger, nor were they leading the mission.