The Obama administration challenged five key military allies Wednesday to take on a greater share of the NATO-led air campaign against Muammar al-Qaddafi's forces, illustrating the strains of a three-month intervention in Libya that has no time frame for an exit.

Defense Secretary Robert Gates delivered the pointed message in a goodbye to his counterparts from NATO ahead of his retirement next month, senior American and British officials said. But none of the nations that were challenged pledged to do more.

The pressure on Germany, Poland, Spain, Turkey and Netherlands comes as the alliance continues with intensified airstrikes on Libya's capital and only a day after President Barack Obama played down any suggestions of a rift with German Chancellor Angela Merkel over her nation's lack of contribution to the war effort.

"Secretary Gates was very blunt," said Liam Fox, the defense secretary of Britain, which along with France has led the mission to protect Libyan civilians from Qaddafi's troops.

Gates said Spain, Turkey and the Netherlands should enhance their limited participation in noncombat operations by joining in strike missions against ground targets, U.S. officials said, speaking on condition of anonymity to discuss internal NATO deliberations. They said Gates pressed Germany and Poland, the two countries not participating at all militarily, to help in some form.

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Secretary of State Hillary Rodham Clinton likely will restate Gates' argument Thursday when NATO nations and Arab governments participating in the air campaign meet in the United Arab Emirates.

"At each meeting, the international pressure is growing and momentum is building for change in Libya," State Department spokeswoman Victoria Nuland said.

Nuland and other officials accompanying Clinton to Abu Dhabi said the time had come to look at the next phase of the Libya situation -- what replaces the despotic Qaddafi regime.

That means the U.S. and its partners will start looking at what fate -- exile, prosecution or some third option -- should befall the leader and his family. They also must consider the parameters for a future cease-fire between rebels and remaining Qaddafi loyalists, and then how to ensure a viable democratic process, according to U.S. officials who said these issues would be the focus of discussions in Abu Dhabi.

Gates, according to officials, said the additional military support wasn't needed to continue air operations for another 90 days, which has been welcomed by all 28 NATO countries. But he said it was a necessary element of fairness in an alliance built on the principle of shared burdens.

Eight NATO members are participating in air strikes in Libya: the U.S., Britain, France, Belgium, Canada, Norway, Denmark and Italy. Denmark and Norway in particular are contributing disproportionately, given the size of their militaries, U.S. officials said, and both are feeling the stress on their aircraft and crews as well as a financial strain.

These stresses, combined with the refusal of some alliance members to participate at all in offensive operations, are one reason U.S. officials believe NATO is excessively dependent on the United States for its advanced military power.

Britain echoed the U.S. concerns.

"There are other countries that have assets which we could be using, not necessarily ground attack, but in support and reconnaissance, in air-to-air refueling and so on," Fox said. "Too many are doing too little."

The U.S. and its allies say the upturn in NATO airstrikes is increasingly pinning Qaddafi into a corner, even as troops loyal to the erratic leader of 42 years lashed back with renewed shelling of the western city of Misrata on Wednesday, killing 10 rebel fighters. Yet NATO's strikes on Wednesday -- at least four strikes during the day after five before dawn -- reinforced the limits to its intervention.

Despite overwhelming aerial power, it has been difficult for the international coalition to end the threat from Qaddafi without overstepping its mission. NATO says it is scrupulously following the U.N. resolution calling for the protection of civilians, and not for the ouster of Qaddafi, who is vowing to fight to the death.

By singling out countries, Gates put longstanding allies on the spot at a time when NATO leaders are emphasizing their solidarity in the Libya mission. NATO Secretary General Anders Fogh Rasmussen told reporters Wednesday the alliance is well on its way to fulfilling the mission and paving the way for a post-Qaddafi period.

But nearly three months after operations started, no one can convincingly say how fast the intervention will be able to end. The opposition holds maybe a third of the inhabitable area of the country, but has struggled to make advances. And as long as Qaddafi's forces remain close to inhabitable areas, the threat to civilians persists.

The U.S. military moved to a secondary role after the initial period of air and naval bombardment that established a no-fly zone over the North African country and opened the door to the NATO-led air campaign.

Obama has declined to put U.S. warplanes back into an offensive role -- aside from a relatively small number of planes targeting Libya's air defenses. But a few weeks ago the U.S. provided nine more aerial refueling planes to enable NATO to accelerate its bombing, the U.S. officials said.

Further U.S. escalation would face sharp scrutiny at home. On Wednesday, Democratic Sen. Jim Webb of Virginia and Republican Sen. Bob Corker of Tennessee unveiled a resolution calling on Obama to seek congressional consent for military involvement in Libya and prohibiting American ground forces in Libya.

The measures builds on a resolution passed by the House last week rebuking Obama for failing to get authorization from Congress before ordering air strikes in March.