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(CNN) -- A U.N. resolution justifies the targeting of Libyan leader Moammar Gadhafi, a senior NATO military official with operational knowledge of the Libya mission told CNN Thursday.

Asked by CNN whether Gadhafi was being targeted, the NATO official declined to give a direct answer. The resolution applies to Gadhafi because, as head of the military, he is part of the control and command structure and therefore a legitimate target, the official said.

NATO has been ramping up pressure on the regime, employing helicopters last weekend for the first time against Gadhafi's forces. Explosions are heard often in Tripoli, evidence of allied air strikes.

NATO began bombing Libya on March 31, under a U.N. mandate to protect civilians who have been targeted by Gadhafi's military.

Resolution 1973 said allied forces could use "all necessary measures" to protect civilians.

NATO Secretary General Anders Fogh Rasmussen said Wednesday it is time to start planning for what to do in Libya after Gadhafi's departure "because Gadhafi's reign of terror is coming to an end."

In statement broadcast on state media, Gadhafi vowed a day before that "we will not surrender," even as NATO airstrikes bombarded his compound in Tripoli.

NATO recently announced its decision to extend its mission in Libya by 90 days.

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