Heavy fighting raged Tuesday in the western Libyan city of Misrata, witnesses said, while a NATO commander complained the alliance was having trouble destroying Moammar Gadhafi's mortars and rockets attacking rebels there and Britain said it would send senior military officers to advise the opposition in the east.

A senior Libyan official, meanwhile, ruled out the possibility of allowing foreign troops to escort humanitarian aid convoys in Libya, saying the government would view such a deployment as a military mission.

Explosions and gunfire were heard in central Misrata, Libya's third-largest city, with clashes between government troops and rebels, said a resident who identified himself only by his given name, Abdel Salam, for fear of retaliation. The city has been besieged by government forces for more than a month.

NATO planes flew over Misrata while the shelling from Gadhafi forces continued, he said, adding that the only targets the alliance hit were radars and air defenses north of the city on Monday night.

NATO Brig. Gen. Mark van Uhm said fighting has been intense in Misrata for the past 10 days, and he said his forces have destroyed more than 40 tanks and several armored personnel carriers there.

"The situation on the ground is fluid there, with ground being won and lost by both sides," van Uhm said at NATO headquarters in Brussels, adding: "Gadhafi's forces have shelled Misrata indiscriminately."

But he cautioned that "there is a limit to what can be achieved by airpower to stop fighting in a city."

"We are doing everything to prevent civilian casualties by our own attacks (while) degrading (Gadhafi's) ability to sustain forces there," he said.

Adm. Giampaolo Di Paola, the chairman of the alliance's military committee, said in Rome that even though NATO operations have done "quite significant damage" to the Libyan regime's heavy weaponry, what Gadhafi has left is "still considerable."

Asked if more NATO air power and bombing are needed, Di Paola said any "significantly additional" allied contribution would be welcome.

Given NATO's humanitarian mandate reflecting the U.N. Security Council resolution on Libya, which does not allow ground forces, "it's very difficult" to stop the regime's firepower on Misrata, he said.

"It's not a conventional war," the admiral said, declining to say just how much of the regime's firepower has been eliminated or put out of action by NATO's operations so far.

"What is significant is we're preventing Gadhafi from using the full potential of his firepower. Unfortunately we're not able so far to deny him use of all his firepower," Di Paola said.

Di Paola said the alliance had "yet to succeed" in neutralizing the mortars and rockets, especially inside Misrata, where it is "very hard" to destroy that firepower without inflicting civilian casualties.

As the allies seek ways to break the battlefield stalemate in the uprising that began in February, British Foreign Secretary William Hague said it will send a team of up to 20 senior military advisers to Benghazi to help organize the country's haphazard opposition forces.

Hague insisted the advisers would not be involved in supplying weapons to the rebels or in assisting their attacks on Gadhafi's forces. He said the advisers would work with British diplomats already cooperating with the National Transitional Council, the political wing of the rebel movement, which has been officially recognized by Italy, France and Qatar.

Britain has said it would not become involved in directly supplying weapons to Libya's rebels; it has already sent non-lethal support, such as 1,000 sets of body armor and 100 satellite phones.

"They will advise the National Transitional Council on how to improve their military organizational structures, communications and logistics, including how best to distribute humanitarian aid and deliver medical assistance," Hague said.

Over the weekend, the U.N. reached agreement with Gadhafi's regime on carrying out aid operations in areas of Libya he controls. A key destination for such aid would be Misrata.

The European Union is drawing up contingency plans for humanitarian aid escorts, who would have a noncombat role, according to the British daily The Guardian. The newspaper quoted unidentified diplomats as saying such plans would not be finalized unless the U.N. Office for the Coordination of Humanitarian Affairs requests such assistance.

But Libyan Deputy Foreign Minister Khaled Kaim, asked about the possibility of foreign troop escorts of aid convoys, said "if there is any deployment of any armed personnel on Libyan ground, there will be fighting, and the Libyan government will not take this as a humanitarian mission" but as a military one.

Asked whether he would rule out such deployment, he told reporters in Tripoli: "Yes, because we are doing our utmost not to resort to such things." He said the Libyan government has repeatedly offered to help humanitarian agencies do their work.

Kaim also said NATO airstrikes have knocked out telecommunications in the central Libyan towns of Sirte, Brega and Ras Lanouf. He alleged phone networks were targeted to help the rebels advance westward, into areas controlled by Gadhafi's forces.

Asked about the strikes, NATO Brig. Gen. Mark van Uhm said that "in order to take out that threat (of pro-regime forces against the civilian population), we make it more difficult for Gadhafi's ability to control his forces."

Concerning the EU's reported contingency plans, he said that "there has been no need for armed escorts until now."

"Until now, it has not been necessary to use armed escorts, and since the port of Misrata is still open, we don't see the need," he told reporters.

World Food Program spokeswoman Emilia Casella says the U.N. agency signed an agreement with the Libyan Red Crescent to establish a humanitarian corridor in western Libya and "we received an indication that the government did not have any objection."

Casella said WFP trucks are already bringing food to feed 50,000 people for one month. The food will be distributed by the Libyan Red Crescent in Tripoli, Zintan, Yefrin, Nalut, Mizda, Al Reiba and Zawiya.

Separately, the U.N. humanitarian chief said she was assured the U.N. would be permitted to visit Misrata and other towns to assess the humanitarian need.

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Michael reported from Cairo. Associated Press writers Frances D'Emilio and Alessandra Rizzo in Rome, Slobodan Lekic in Brussels and contributed to this report.