The takeover comes at a sensitive moment in the war between the rebels and loyalist forces. Moammar Gadhafi's forces have nearly reversed recent rebel gains, but more support could be on the way.
President Barack Obama has reportedly authorized covert operations to help the rebels, and the CIA has already sent in a small group of operatives.
The president has not directed arms be provided to them, but it is being debated.
Britain and the U.S. believe that existing U.N. Security Council resolutions on Libya could allow for foreign governments to arm the rebels, despite an arms embargo being in place.
The NATO secretary-general said he has "taken note of the ongoing discussions in a number of countries but as far as NATO is concerned ... we will focus on the enforcement of the arms embargo."
In the meantime, congressional hearings on the U.S. involvement in Libya are scheduled for Thursday. Lawmakers who received closed-door briefings on Wednesday said they were told that no decision has been made about arming rebels yet.
A rebellion against Gadhafi's 42-year rule erupted last month, and international forces including the U.S., France and Britain stepped in March 19, just as it appeared Gadhafi was on the verge of crushing the revolt.
The Associated Press contributed to this story.