At a summit of NATO nations that opened here Thursday, U.S. officials played down emerging rifts among allies and rebuffed calls from within NATO for its members to commit more forces to the military operation in Libya.

Since the United States turned over command of the airstrikes in Libya to NATO at the end of March, there has been growing criticism from some in the coalition — particularly France and Britain — that other allies need to do more to help Libya’s rebel opposition battle Libyan leader Moammar Gaddafi.

French Foreign Minister Alain Juppe said he asked Secretary of State Hillary Rodham Clinton in a closed-door meeting Thursday whether the United States could contribute additional fighter planes to the effort but did not receive an encouraging response.

“I got the sense that the Americans will stick to their same line,” Juppe said. “That is, to maintain their current policy of intervening with forces as they are needed, depending on the situation and where the assets they have are particularly useful.”

U.S. officials have pushed back against such demands, saying that NATO has not yet asked the United States directly for additional assets and pointing out that they are already supplying many support-type planes. They also say they believe other countries will eventually come forward.

U.S. officials had hoped to use the two-day NATO summit to bridge such emerging differences in the coalition.

“Gaddafi is testing our determination,” Clinton told other foreign ministers during the summit. “As our mission continues, maintaining our resolve and unity only grows more important.”

At a Thursday morning news briefing ahead of the summit, U.S. officials insisted that NATO commanders in charge of the operation have everything they need.

“If the commanders feel they need more capability, the commanders will ask for more capability. That’s not what they are doing so far,” said a senior U.S. administration official who spoke on the condition of anonymity.

During the Berlin meeting, however, NATO Secretary General Anders Fogh Rasmussen said commanders had indeed sought more military assets, specifically requesting equipment capable of precision attacks on ground forces.

According to Rasmussen, NATO’s supreme allied commander for Europe, Adm. James Stavridis, told foreign ministers that while NATO has the overall assets required for the mission, its military needs have changed with recent shifts in Gaddafi’s strategy.

“We had great initial success bombing his tanks, but we’re encountering problems now that Gaddafi’s moving his heavy armor closer to civilian populations,” said a Western diplomat involved in the discussions. According to that official, NATO now needs about eight more planes capable of precision bombing, such as the U.S. F-15 or F-16 or similarly equipped aircraft from other countries.

Spain and Italy have planes helping to enforce the no-fly zone, but both countries said Thursday they did not plan to step up their role to include ground strikes.

Rasmussen declined to single out the United States as the country he and NATO commanders hoped would provide the additional assets now needed, and said, “I’m confident that nations will step up to the plate.”

In Libya, meanwhile, Gaddafi chose the same day and, according to some reports, the very hour NATO ministers were meeting to ride around Tripoli in an open-top sport-utility vehicle, pumping his fist defiantly, in an act broadcast on Libyan state TV.

And 130 miles to the east, the besieged city of Misurata came under heavy rocket and artillery fire Thursday morning, according to residents and doctors there. Explosions rocked the city — the only opposition foothold left in western Libya — as Gaddafi loyalists intensified their attacks and the city’s port was closed amid the violence, port authority officials in the rebel stronghold of Benghazi said.

Misurata has been under siege for seven weeks as Gaddafi’s forces have tried to regain complete control of the remote city, but residents have held out despite sniper attacks and artillery shelling that have killed hundreds.

Benghazi port authority officials said aid organizations’ boats that had been shuttling medicine and supplies from Benghazi to Misurata, as well as evacuating the wounded, were unable to dock Thursday because of shelling.

At the port in Benghazi, ships were also being loaded with rockets and guns to resupply rebels even as news spread of Misurata’s port closing. One boat destined for Misurata was being loaded with bulletproof vests, helmets and ammunition in crates labeled in English as having come from Qatar.

Misurata’s hospitals were overflowing Thursday, with at least 13 people dead and 50 wounded in the latest attacks, said Shaymaa Najil, an Iraqi doctor volunteering with the Red Cross there.

“We don’t have enough medicine, doctors, beds or tools,” she said. “Every day is the same now. We wake up to bombing and go to sleep to bombing.”

In her speech in Berlin, Clinton spelled out the actions she deemed essential for the Libya mission, including improving coordination between NATO commanders and rebel forces — some of whom have been killed by friendly fire from NATO planes — and intensifying financial and diplomatic international pressure on Gaddafi. Planning for a post-Gaddafi Libya must also begin, she said.

U.S. officials said Clinton also met with Turkish officials on Thursday and talked about creating “an exit path” for Gaddafi to step down and leave Libya.

Turkey maintains a unique position as the only country in NATO that still has a functioning embassy in Tripoli, as well as a consulate in Benghazi.

A senior Turkish official said his government hoped to use the Berlin meeting to come up with a diplomatic solution, which would include establishing direct communication with Gaddafi, making arrangements for his exit and ensuring that the opposition council sets up a new government that represents all tribes and groups.

Turkish Prime Minister Recep Tayyip Erdogan has maintained telephone contact with Gaddafi and his sons since before the start of the military operations, Ibrahim Kalin, Erdogan’s senior adviser, said in an interview late Wednesday. On each occasion, Erdogan “urged Gaddafi to leave peacefully,” Kalin said.

“If the Berlin meeting produces results,” Kalin said, a cease-fire could be declared within days. Although he acknowledged the difficulty of dealing directly with Gaddafi, he said the international community needs to “open a line of communication” with the Libyan leader, through Arab interlocutors or Turkey itself, to assure him he has a viable exit and convince him that he has no other option.

Fadel reported from Benghazi, Libya. Correspondent Simon Denyer in Tripoli and staff writer Karen DeYoung in Washington contributed to this report.