TRIPOLI, Libya — Moammar Gadhafi stood defiant Tuesday in the face of the heaviest and most punishing NATO airstrikes yet – at least 40 thunderous daylight attacks that sent plumes of smoke billowing above the Libyan leader’s central Tripoli compound.

In late afternoon and as the strikes continued, Libyan state television broadcast an audio address from Gadhafi, who denounced NATO and the rebels challenging his rule. He vowed never to surrender. “We will not kneel!” he shouted.

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.

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.

It couldn’t be confirmed whether Gadhafi’s 10-minute speech was a live phone call or an audio recording, but it appeared to take state television by surprise. The sound was hastily adjusted to make it louder.

“We will not surrender: we only have one choice – to the end! Death, victory, it does not matter, we are not surrendering!”

The sharp escalation in NATO strikes comes as the U.S. and its allies step up efforts to break the virtual stalemate that has prevented both sides from achieving an outright victory.