BRUSSELS - NATO is showing some strains after nine days in charge of the allied military operation in Libya.

First, the military alliance holds its fire as Libyan leader Moammar Gadhafi's forces advance a full 100 miles (160 kilometres) into rebel-held territory. Then NATO accidentally opens fire on Libyan rebels in tanks -- a top general says NATO didn't know the rebels had any -- even though footage of rebels with tanks had been on YouTube for weeks.

NATO's leadership of the air attack campaign is coming under increasing criticism for mistakes and ineffectiveness -- particularly in comparison with the previous American-led effort.

"This is something new. We haven't had a significant military operation in which the Americans have taken a back seat for quite some time," Malcolm Chalmers, a professor of defence at London's Kings College, said Friday. "It really is unclear whether the Europeans can rise to that challenge."

The NATO bombing of a rebel convoy on Thursday, in which five people died and a number of rebel tanks were destroyed, appears to have crystalized the perception -- to outsiders, at least -- that the alliance is running a bumbling campaign.

But as the rebels angrily accused the alliance of mistakes and neglect, NATO's frustrated leaders refused to apologize Friday for the bombing of the tanks. And NATO commanders, in turn, are frustrated that the rebels see NATO as their proxy air force, rather than a force to protect civilians in Libya.

Analysts say the recent developments have brought into sharp relief the confusion, ambiguity and constraints of NATO's mission in Libya, which began nine days ago when the North Atlantic alliance assumed command from a U.S.-led coalition that launched the air war on March 19.

There is significant ambiguity about the scope and objective of the mission. The U.N. resolution under which the alliance operates requires it to protect civilians from Gadhafi's forces while remaining impartial.

"There's a very difficult trade-off for NATO here," Chalmers said. "If they wait until they're absolutely certain that they've got the targets right and that there are no civilians, Gadhafi's forces will have vanished in the confusion by then."

Adding to NATO's woes, the U.S. -- which handed off its leadership role March 31 -- then went further and halted its combat role earlier this week. That move is depriving NATO of certain kinds of aircraft that could prove useful in some of the close urban warfare battles between forces loyal to Gadhafi and rebels bent on his ouster.

NATO acknowledged Friday that its airstrikes had hit rebels using tanks to fight government forces in eastern Libya, saying it thought only Gadhafi regime forces had used heavy armoured vehicles.

Yet if NATO did not know, that seems extraordinary: Video and photos from the start of the uprising against Gadhafi's rule a month ago showed that some Libyan armoured units had changed sides in the early stages of the rebellion, bringing their equipment with them.

On Friday, British Rear Adm. Russell Harding -- deputy commander of the NATO operation -- also said it was difficult for allied pilots to distinguish between rebels and regime troops engaged in a series of advances and retreats between the eastern coastal towns of Brega and Ajdabiya.

"I am not apologizing (for the bombing)," Harding told reporters in Naples, where the alliance's operational centre is located. "The situation on the ground was and remains extremely fluid, and until yesterday we did not have information that (rebel) forces are using tanks."

NATO's Secretary-General Anders Fogh Rasmussen expressed regret over the rebels' loss of life, but he too offered no apology.

Complicating matters further for NATO, ground fire over the Libyan battlefields remains a serious threat to any jet making low-level passes -- a must for pilots trying to positively identify enemy forces in a fast-changing situation.

Although the first U.S.-led strikes on Libyan targets quickly destroyed most of Gadhafi's fixed surface-to-air missile emplacements and the radars that control them, Gadhafi's forces are believed to have hundreds of automatic cannons and shoulder-launched rockets -- including sophisticated Russian-built Iglas -- that can easily down planes like A-10 Thunderbolts or AC-130 gunships at low altitudes.

NATO learned this the hard way during the 1999 war in Kosovo, where a number of its attack jets were struck by ground fire and had to make emergency landings on nearby alliance-held airports. Commanders then ordered the pilots not to descend lower than 5,000 metres (15,000 feet), keeping them outside the killing range of guns but drastically reducing the effectiveness of their bombing attacks on Serbian ground forces.

Now, NATO jets are again operating mainly at higher altitudes, where Iglas and Gadhafi's pickup-mounted 37mm and 20mm guns cannot reach them.

Harding said Friday that NATO jets had conducted 318 sorties and struck 23 targets across Libya in the past 48 hours. They have flown over 1,500 sorties since assuming overall command.

The jets have destroyed Gadhafi's anti-aircraft missile defences, T-72 tanks and ammunition dumps, Harding said. The NATO attacks have also targeted Gadhafi's loyalist forces in the besieged city of Misrata, where rebels continue to hold out.

But critics have questioned NATO's limited mandate of only protecting civilians directly threatened by Gadhafi's troops, rather than trying to eliminate the threat completely by destroying the strongman's regime.

"By not striking at the regime from the outset, Gadhafi was granted the initiative to embed his forces in urban settings hiding behind human shields in a form of guerrilla warfare," said Barack Seneer, a Middle East expert at the Royal United Services Institute, a British military think-tank .

"A no-fly zone is not equipped to contend with guerrilla warfare or with a stalemate that places rebels and loyalists at close proximity with one another," he said.

U.S. Defence Secretary Robert Gates said NATO is flying about the same number of combat missions in Libya as when the U.S. was part of the strike mission -- so it should be no surprise that they provide only limited help to the Libyan rebels.

"With not having our own people on the ground, without having forward air controllers and observers and so on, and with the pilots trying to go out of their way to avoid civilian casualties, obviously it becomes more difficult to support ground operation," he told reporters in Mosul, in northern Iraq.

Gates also said Gadhafi's forces using more civilian vehicles and clothing to blend in with rebel forces, which makes it even more complicated for NATO's combat pilots to distinguish friend from foe.

Analysts suggest that neither side in Libya can deliver a decisive blow against the other anymore, and say the war has turned into a stalemate that could last for many months.

"The initial military operation achieved its objective of preventing a massacre of rebels and civilians in Benghazi," Chalmers said. "But NATO inherited a much messier situation, and we are now entering a period in which politics and not the military will have to play a leading role."