Libyan Rebels Advance to Sirte as NATO Assumes Command

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WATCH Rebel Advances in Libya

Air raids targeting the Libyan city of Sirte tonight aided rebels advancing towards the city, a regime stronghold, while it was confirmed that NATO will now be assuming the entire mission in the war-torn country, including all air strikes and the civilian protection mission.

Taking Moammar Gadhafi's hometown Sirte, which lies halfway between the rebel-held east and the government-controlled west, would be a major coup for the rebels who are quickly advancing toward the capitol city of Tripoli.

Entrances to Sirte have been mined, according to The Associated Press.

Watch ABC News for President Obama's speech on U.S. involvement in Libya, anchored by Diane Sawyer and George Stephanopoulos, live, Monday at 7:30 p.m. ET

Earlier today, rebels regained two key oil complexes along Libya's coastal highway. Their westward march towards Sirte mirrors their earlier advancement towards the capitol, but this week they had powerful air forces bombarding Gadhafi's military and clearing a path.

Shortly after nightfall, an air raid on Tripoli began. A reporter for the AP said nine loud explosions and anti-aircraft fire was heard in the Libyan capitol.

Around the same time, foreign journalists in Sirte began reporting heavy explosions and warplanes flying overheard, the AP reported. Libyan state television also confirmed air raids in Sirte and Tripoli.

Fighting began in the contested city of Misrata -- which lies between Sirte and Tripoli -- as residents reported that pro-Gadhafi forces were firing on residential areas.

Meanwhile, it was confirmed that NATO will assume command of all aerial operations in Libya from the U.S.-led force.

According to Secretary-General Anders Fogh Rasmussen, NATO jets had already begun enforcing the no-fly zone today.

"NATO allies have decided to take on the whole military operation in Libya under the U.N. Security Council resolution," Fogh Rasmussen said in a statement. "NATO will implement all aspects of the U.N. resolution. Nothing more, nothing less."

According to a senior Obama administration official who spoke on condition of anonymity, NATO will now assume the entire mission of implementing UN Security Council resolution 1973, which demands an immediate ceasefire in Libya, including an end to the current attacks against civilians.

This fulfilled President Obama's promise to take the leading edge with America's unique capabilities, then quickly turn over authority to NATO, the official said.

More countries should be contributing assets to the operation, the official said, adding that not every country will have to take part in every aspect of the mission. While some may contribute to enforcing the arms embargo or the no-fly zone only, others may participate in bombings to protect civilians.

NATO will lead all of it, the official confirmed, stressing that the mission is to strike at anyone who targets civilians, not to support the Libyan rebels.

NATO commanders will have the authority to pick bombing targets and won't have to seek permission from a political body first, the official said.

Defense Secretary Robert Gates maintained said he could not provide a timetable for operations in Libya, according to the AP.

Speaking with ABC News' Jake Tapper on "This Week" today, Gates said Libya presented no immediate threat to the United States.

"No, no -- it was not a vital national interest to the United States, but … the engagement of the Arabs, the engagement of the Europeans, the general humanitarian question, that was at stake," Gates said.

"For all practical purposes, the implementation of a no-fly zone is complete. Now it will need to be sustained, but it can be sustained with a lot less effort than what it took to set it up," Gates said.

Appearing alongside Gates on "This Week," Secretary of State Hillary Clinton defended the mission in Libya while downplaying its length.

"I think that this had a limited timeframe, a very clearly defined mission, which we are in the process of fulfilling," Clinton said.

ABC News' Kirit Radia contributed to this report