By Maria Golovnina and Michael Georgy

TRIPOLI (Reuters) - Western warplanes silenced Muammar Gaddafi's artillery and tanks besieging rebel-held Misrata in western Libya on Wednesday after a U.S. admiral warned his armor was the next target.

Breathing defiance, Gaddafi earlier said Western powers who carried out a fourth night of air strikes on Libya to protect civilians under a U.N. mandate were "a bunch of fascists who will end up in the dustbin of history."

Gaddafi's tanks had kept up the shelling of Misrata, killing dozens of people this week, and residents said a "massacre" was taking place with doctors forced to treat the wounded in hospital corridors. Snipers fired at people from rooftops.

"The allied planes bombed twice so far. At 12:45 (6:45 p.m. ET Tuesday) this morning and then again less than two hours ago," a resident, Saadoun, told Reuters by telephone from Misrata.

"They (pro-Gaddafi forces) haven't fired a single artillery (round) since the air strike."

At least two explosions were heard in the Libyan capital Tripoli before dawn on Wednesday on a fourth night of strikes, Reuters witnesses said. The roar of a warplane was heard above the city followed by a barrage of anti-aircraft gunfire.

Prior to the Misrata strikes, Rear Admiral Peg Klein said warplanes would be sent out to attack Gaddafi's tanks.

"Some of those cities still have tanks advancing on them to attack the Libyan people," said Klein, commander of the expeditionary strike group aboard the USS Kearsarge off Libya.

"We are authorized, and the president made the nexus between the Security Council resolution and what he considers our legal mandate to attack those tanks. So that is the type of target that our strike aircraft will go at."

While Western air power has grounded Gaddafi's warplanes and pushed back his forces from the brink of rebel stronghold Benghazi, his forces have been besieging Libyan holdouts by rebels fighting to overthrow his 41-year rule.

In the east of this oil-producing north African desert state, disorganized and badly equipped rebels have failed to capitalize on air strikes and are pinned down.

The fighters have been unable to dislodge Gaddafi's forces from the key junction of Ajdabiyah in the east bringing a big risk of stalemate on the ground, security analysts say.

"NO SURRENDER"

"We will not surrender," Gaddafi had told supporters forming a human shield to protect him at his Tripoli compound, which came under attack in 1986 from the U.S. Reagan administration and once again in the current round of air strikes.

"We will defeat them by any means ... We are ready for the fight, whether it will be a short or a long one ... We will be victorious in the end," he said in a live television broadcast, his first public appearance since the air strikes began.

The Libyan government denies its army is conducting any offensive operations and says troops are only defending themselves when they come under attack.

The siege of Misrata, now weeks old, had become increasingly desperate, with water cut off for days and food running out, doctors operating on patients in hospital corridors and many of the wounded left untreated or simply turned away.

"The situation in the local hospital is disastrous," said a Misrata doctor. "The doctors and medical teams are exhausted beyond human physical ability and some of them cannot reach the hospital because of tanks and snipers."

It was impossible to independently verify the reports.

The rebel effort in the desert scrub in the east was stuck outside Ajdabiyah, 150 km (90 miles) west of Benghazi.

There has been no movement on the strategic town since Gaddafi's remaining tanks holed up there after the government's armored advance along the open road to Benghazi was blown to bits by French air strikes on Saturday night.

HIDING IN THE DUNES

Hiding in the sand dunes from the tank fire coming from the town, the rebels are without heavy weapons, leadership, communication, or even a plan.

On Tuesday, groups of fighters lounged around, chatting and smoking. This was the spearhead of the counter-offensive.

When asked who was in command, one fighter, Mohamed Bhreka, shrugged and said: "Nobody is. We are volunteers. We just come here. There is no plan."

Their heavy machine guns were bolted to the back of pick-up trucks and there was a good supply of assault rifles. But some just had knives or iron bars. Field radios were not to be seen.

Fighters on the frontline of the uprising against Gaddafi said they had lost the heavy weapons vital to take on tanks.

It remains to be seen whether the rebels' bravado and faith in God are enough to take towns and advance toward their target of capturing Tripoli.

Western warplanes have flown more than 300 sorties over Libya and more than 162 Tomahawk cruise missiles have been fired in the mission to protect civilians against government troops.

Keen to deliver the mission's result, U.S. President Barack Obama said the allies should be able to announce soon that they have achieved the objective of creating the no-fly zone.

But, he said, Gaddafi would present a potential threat to his people "unless he is willing to step down."

"We will continue to support the efforts to protect the Libyan people. But we will not be in the lead," Obama said.

Facing questions at home about the Libyan campaign, duration and cost, Obama wants the United States to give up operational control of enforcing the no-fly zone within days.

Obama spoke with French President Nicolas Sarkozy and British Prime Minister David Cameron on Tuesday and they agreed NATO should play an important role in enforcing the Libyan no-fly zone, the White House said.

France had been against a NATO role for fear of alienating Arab support. Turkey also opposed the alliance taking command as it said air strikes had already overstepped what was authorized by the United Nations. U.S. officials said both countries' objections had been overcome.

NATO COMMAND

The plan is for NATO's command structure to be used for the operations under the political leadership of a "steering body" made up of Western and Arab nations who are members of the alliance policing Libya's skies, diplomats said.

French Foreign Minister Alain Juppe said on Tuesday foreign ministers of countries taking part in the military action were set to meet in the coming days to create a clear political structure for operations.

"I've proposed with the agreement of our British colleagues that we set up a political structure to guide operations, involving foreign ministers from countries that are taking part and from the Arab League," Juppe told the French parliament.

While Gaddafi scoffed at the West in his latest speech, U.S. Secretary of State Hillary Clinton said the Libyan leader and his allies may be exploring exile options, although it was unclear if he would seriously contemplate stepping aside.

"Some of it is theater," Clinton told ABC News in an interview, saying Washington was aware of people reaching out "allegedly on Gaddafi's behalf" to try to assess their options.

"A lot of it is just the way he behaves. It's somewhat unpredictable. But some of it, we think, is exploring. You know, what are my options, where could I go, what could I do. And we would encourage that," she said.

Clinton also said the U.S. government had received unconfirmed reports that at least one of Gaddafi's sons may have been killed in air strikes. She said the "evidence is not sufficient" to confirm the reports, but added it was not U.S. forces that would have killed him.

(Reporting by Mohammed Abbas and Angus MacSwan in Benghazi, Maria Golovnina and Michael Georgy in Tripoli, Hamid Ould Ahmed and Christian Lowe in Algiers, Tom Perry in Cairo; David Brunnstrom in Brussels, Phil Stewart in Moscow, Andrew Quinn in Washington; Writing by Peter Millership; Editing by Giles Elgood)