TRIPOLI, Libya — NATO airstrikes rumbled over the Libyan capital on Wednesday, apparently targeting Moammar Gadhafi's sprawling compound in a new round of daylight raids. NATO's top official said Gadhafi's fall was only a matter of time and reiterated that the alliance would send no ground troops.

At least four daytime airstrikes sounded over the capital, after five strikes before dawn.

It was not immediately clear what was targeted. However, NATO strikes appear to be repeatedly pounding the same set of targets: the sprawling Gadhafi compound in central Tripoli, a series of government buildings and on the city's outskirts, radar installations and military bases.

In Brussels, NATO Secretary-General Anders Fogh Rasmussen said: "For Gadhafi, it is no longer a question of if he goes but when he goes."

But when Gadhafi goes, Rasmussen said, it would be up to the United Nations and not NATO to usher Libya peacefully toward democracy.

"We do not see a lead role for NATO in Libya once this crisis is over," he said. "We see the United Nations playing a lead role in the post-Gadhafi, post-conflict scenario."

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, also stressed that message during a visit to Cairo, noting the recent NATO decision to extend the Libyan mission for 90 days, into late September.

"I think it's very clear that NATO is very committed to this mission and committed to providing the kind of protection for the Libyan people that it has when it took the mission on and to focusing on a way to see Gadhafi out the door," he said.

He added that President Barack Obama "has been very clear and remains very clear that this will not involve boots on the ground from the United States perspective."

Alliance officials warned for days that they were increasing the scope and intensity of their air campaign to oust Gadhafi after more than 40 years in power. NATO is backing the rebel insurgency, which has seized swaths of eastern Libya and pockets in the regime's stronghold in the west since it began in February, inspired by uprisings elsewhere in the Arab world.

British and French attack helicopters struck for the first time inside Libya over the weekend and the alliance on Tuesday flew 66 "strike sorties" on Tuesday, its most intense barrage yet in the conflict.

Some 6,850 people, nearly all of them Libyans, have streamed across the border from Libya to Tunisia since Monday to flee the NATO raids as well as fighting between the rebels and government forces, according to the Tunisian Defense Ministry.

Wing Commander Mike Bracken at NATO's Libya operations headquarters in Naples told The Associated Press there has been "increased tempo over recent days over Tripoli" as the alliance seeks to further weaken Gadhafi's military.

But he stressed that "Gadhafi as an individual has not been a target and won't be a target."

In Benghazi, the de facto rebel capital, Spanish Foreign Minister Trinidad Jimenez became the latest European official to visit and bolster the opposition forces.

Fogh Rasmussen has said he will use the two-day meeting of alliance defense ministers, which starts Wednesday at NATO's Brussels headquarters, to push for broader participation by allies. He wants more countries from the 28-nation alliance to share the costs and risks involved in the campaign.

A defiant Gadhafi vowed Tuesday to fight to the death.

"We will not surrender: we only have one choice — to the end! Death, victory, it does not matter, we are not surrendering!" Gadhafi said in an audio broadcast on state television.

Gadhafi was last seen in a brief appearance on state television in late May. He has mostly been in hiding since NATO strikes in April targeted one of his homes. Libyan officials said one of his sons, Saif al-Arab, and three of his grandchildren were killed in that strike.

Corder contributed from Brussels. Associated Press Writer Hadeel Al-Shalchi contributed from Benghazi, Libya.