White House forced to defend its role in Libya
In a 32-page report sent to an increasingly sceptical Congress, the administration laid out its justification. The document also outlined the cost to US taxpayers, noting it will top $1 billion (600?million) by early September.
The legal defence comes amid growing disquiet in Washington over the conflict, which was entered into without Congress's approval.
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Hide AdUnder a Vietnam-era law, troops can be deployed without wider authorisation, but must be withdrawn again within 60 to 90 days.
Earlier this week, Republican House speaker John Boehner wrote to Mr Obama to point out that the three-month period runs out on Sunday.
The White House responded: "US operations do not involve sustained fighting or active exchanges of fire with hostile forces, nor do they involve US ground troops."
"We are acting lawfully," Harold Koh, the State Department's legal adviser said.
Mr Boehner's spokesman accused the White House of using "creative arguments".