BERLIN — The United States resisted pressure Thursday to return its warplanes to the frontline in Libya even as NATO vowed to keep bombing Moamer Kadhafi's forces and pressed the strongman to quit.

As NATO foreign ministers meeting in Berlin struggled to heal a rift over the mission, rebels said Kadhafi forces shelled the city of Misrata, killing 13 people, while blasts rocked Tripoli as jets screamed over the capital.

The ministers issued a joint statement calling on Kadhafi to leave power, and they vowed to maintain "a high operational tempo" against regime targets and "exert this pressure as long as necessary."

Despite the show of unity, the allies remained divided over French and British calls to contribute more jets to the mission. Nearly a month of coalition strikes have failed to shift the balance of power so far.

French Foreign Minister Alain Juppe made a personal appeal to US Secretary of State Hillary Clinton for Washington to resume major air raids in Libya, but he said his plea was rebuffed.

"I told her we needed them back, we would have liked them to return," Juppe said, adding that Clinton said US planes would continue to fly on a case-by-case basis.

Washington pulled back around 50 combat planes from Libyan operations last week after handing over control of the mission to NATO, although since then they took part in some missions to take out Kadhafi's air defence systems.

With nearly 100,000 US troops fighting a grinding war in Afghanistan, President Barack Obama's administration decided to move into a back-up role in Libya and leave the major fighting to its European and Canadian allies.

"For our part, the US is committed to our shared mission. We will strongly support the coalition until our work is completed," US Secretary of State Hillary Clinton told her counterparts during a working lunch.

"We are also sharing the same goal which is to see the end of the Kadhafi regime in Libya," she said earlier at a meeting with German Chancellor Angela Merkel.

A senior US official said that the United States was performing one-quarter of overall missions and that it saw no need to do more on the military front.

"We have said all along that we want to see allies step up and that we are certainly doing at least our fair share," the official said.

Other NATO allies in Europe also brushed aside the Franco-British pressure to do more.

NATO Secretary General Anders Fogh Rasmussen said the alliance's military commander in Europe, Admiral James Stavridis, told the ministers that more high-precision planes were needed to safely hit targets hidden in urban areas.

"To avoid civilian casualties we need very sophisticated equipment, so we need a few more precision fighter ground-attack aircraft for air-to-ground missions," Rasmussen told a news conference after a working lunch.

"I'm confident that nations will step up to the plate," Rasmussen added, but he admitted that he had not received any "specific pledges" although "I've heard indications that give me hope."

The senior US official said Stavridis found a 10 percent shortfall in air power, but that this amounted to around eight planes which were readily available.

Only six out of 28 nations are conducting air strikes, while France and Britain carry out half of them. The other half are conducted by Belgium, Denmark, Norway and Canada.

Danish Foreign Minister Lene Espersen piled pressure on allies to do more.

"Denmark is making a huge contribution at the moment and I think it is relevant to discuss burden sharing to put pressure on those countries that haven't started to contribute yet," she told reporters.

But Spanish Foreign Minister Trinidad Jimenez said her country would not step up its contribution.

The allies found common ground over Kadhafi's future, as they backed a call made by the international contact group on Libya which met in Qatar on Wednesday for Kadhafi to step down.

"We welcome the outcome of the first meeting of the contact group which took place yesterday in Doha and strongly endorse its call for Kadhafi to leave power," they said in a statement.

German Foreign Minister Guido Westerwelle, whose country shocked allies by refusing to back the UN resolution authorising the military operation, said NATO was united by a common goal: "We want a free and democratic Libya. The dictator Kadhafi, who started a civil war against his own people, must go."