Embattled Libyan leader Muammar Gaddafi vowed to defeat the NATO alliance's attempt to oust him from power, according to an audio speech aired by state-run television Friday evening.

Supporters of Libyan leader Muammar Gaddafi attend a gathering in Tripoli, Libya, on June 17, 2011, after Gaddafi vowed to defeat the NATO alliance's attempt to oust him from power. [Xinhua/ AFP Photo]

"This is the first time they are facing an armed nation of millions of people," he said. "They will be defeated, the alliance will be defeated."

The station said Gaddafi was speaking in "a telephone call on June 17."

His defiant speech came as troops loyal to him and rebels exchanged heavy fire near the western cities of Zlitan and Misrata, where the rebels are trying to break a months-long siege and push deeper into government-held territory east of the capital.

Loud explosions shook Tripoli just hours before his speech, as NATO warplanes constantly overflew the Libyan capital.

The explosions sent a thick cloud of black smoke rising high into the air but there were no reports about casualties. In the past few weeks, NATO forces have intensified strikes against Tripoli and neighboring areas.

Also on Friday, NATO spokeswoman Oana Lungescu dismissed Gaddafi's offer to hold internationally supervised elections.

"You may have seen yesterday's mention of possible elections in Libya within three months by Gaddafi's son...It's hard to image after 41 years in which Gaddafi abolished elections, constitutions, political parties, trade unions that overnight a dictator would turn into a democrat," she told a press briefing at NATO headquarters.

"So what we and the international community want to see is action rather than words," she said.

Gaddafi's son, Saif al-Islam Gaddafi, told Italian daily Corriere della Sera in an interview published Thursday that elections could be held within three months, with transparency guaranteed by the presence of international observers.

He said his father would be ready to step aside if he lost the election but would not go into exile.

However, Libyan rebels have rejected the offer, saying "the only choice for Gaddafi is to step down."

"Gaddafi's government has lost legitimacy, so he is not supposed to ask for elections and we will not negotiate with him," rebels spokesman Jalal al-Gallal told Xinhua by phone Thursday.

"The international community has been calling on Gaddafi to pull back his troops and allow humanitarian aid to reach the Libyan people," he said.

The United States said Thursday it was "a little late" for Gaddafi to broach any proposal as his days "were numbered."

"It's a little late for any proposal by Gaddafi and his circle for democratic change. It's time for him to go," U.S. State Department spokeswoman Victoria Nuland told reporters.

She said the pressure the international community had brought on Gaddafi and his government was "having an effect," with more than 50 senior-level diplomatic and government officials having defected.