Nato defence ministers are meeting in Brussels to review the results of two months of air strikes in Libya, as the alliance steps up its campaign. More explosions were heard overnight in the capital, Tripoli, after a day of intensified bombardment on Tuesday. Libyan leader Col Muammar Gaddafi has vowed to remain in the country "dead or alive", saying martyrdom would be a "million times better" than surrender. He urged supporters to defy Nato and gather at his Tripoli compound. Meanwhile, China and Russia are involved in separate diplomatic efforts to try to end the conflict. Libyan Foreign Minister Abdul Ati al-Obeidi has been visiting Beijing, and a Russian envoy is in eastern Libya's rebel stronghold of Benghazi to meet opposition leaders. Daytime raids While Wednesday's Nato meeting is likely to centre around maintaining the pressure on Col Gaddafi, it will also focus on what happens when the air campaign ends and during any subsequent transition period, says BBC diplomatic correspondent Jonathan Marcus at the bloc's Brussels headquarters. Last week, Nato agreed to extend its air operations over Libya for a further 90 days, as it increased the scope and intensity of its UN-mandated campaign to protect civilians. Since then, British and French attack helicopters have gone into action and command centres in Tripoli have been pounded. On Tuesday Nato carried out its heaviest daytime raids of its nine-week campaign on what it said were command and control centres in and around the capital, with more than 20 air strikes by low-flying jets. With the increased intensity of its attacks, Nato is sending a clear message to the Libyan leader that the strikes will continue until he hands over power, says the BBC's Wyre Davies in Tripoli. The Libyan government acknowledged that military installations belonging to the Republican Guard had been targeted, while Libyan television reported Col Gaddafi's Bab al-Aziziya compound had also been hit. The whereabouts of the Libyan leader remain unknown, says our correspondent in Tripoli; Col Gaddafi and his sons reportedly spend each night in different locations to evade the Nato air strikes. But he remains defiant, reportedly rejecting an African Union offer to take refuge in an unnamed African nation. In an audio address on Libyan state TV late on Tuesday, the Libyan leader called on his supporters to rally at Bab al-Aziziya, pledging that the Libyan people would soon defeat their enemies. "The Libyan people will march, in the direction of the east or the west, or to any place where there are armed gangs to strip them of their arms without fighting," he said. Addressing Nato, he added: "Your planes will not be able to stop these marches of the millions, nor will the armed gangs that you support be able to resist for even a minute in the face of these marches". But US President Barack Obama said there had been "significant" progress in Nato's mission to protect Libyan civilians, hundreds of whom have been killed since anti-government protests began in mid-February. "What you are seeing across the country is an inexorable trend of the regime forces being pushed back, being incapacitated," he told a news conference after talks with German Chancellor Angela Merkel. "I think it is just a matter of time before Gaddafi goes." On Tuesday, the head of the High-Level African Union Panel on Libya, Mauritanian President Mohamed Ould Abdel Aziz, also said Col Gaddafi's departure had "become necessary". Libyan Labour Minister al-Amin Manfur became the latest senior Libyan official to defect, diplomats with Libya's mission to the UN in Geneva said. Staff in Libya's UN mission in Geneva collectively defected in February. Mr Manfur, who had been representing Libya at the annual meeting of the International Labour Organisation, was said to be on his way to the rebel stronghold of Benghazi. Source: BBC