Correction: An earlier version of this article said that two journalists working in Libya, Tim Hetherington and Chris Hondros, had been reportedly killed in an artillery attack. It remains unclear how the journalists were killed, although they are believed to have come under attack by mortars or rocket-propelled grenades.

The United States and its allies have entered a new stage of involvement in Libya, sending assistance and advisers directly to opposition military forces, which have been unable to break Libyan leader Moammar Gaddafi’s stranglehold over much of the country despite help from NATO airstrikes.

France and Italy said Wednesday that they would join Britain in dispatching military advisers to assist the inexperienced and disorganized rebel army, primarily in tactics and logistics. President Obama authorized sending $25 million worth of nonlethal equipment, including body armor, tents, uniforms and vehicles.

The assistance appeared to stretch the definition of the “civilian protection” mandate contained in last month’s U.N. resolution authorizing foreign intervention in Libya. The allies said their efforts were indirectly achieving that objective, because the rebel force was best-positioned on the ground to protect Libyans from attacks — or the threat of attacks — by Gaddafi loyalists.

The rebel-held western Libyan city of Misurata continued to be the focus of the fighting. Among those killed in the violence were photographers Tim Hetherington, a British American, and American Chris Hondros, reportedly from an attack by Gaddafi forces, and two other Western journalists were wounded. NATO said its warplanes struck government targets on the outskirts of the besieged city, as well as around Tripoli, the capital.

The arrival of European military advisers and U.S. uniforms is unlikely to rapidly change the trajectory of the conflict, however, and NATO and its Arab partners in the Libya operation continue to count on their economic and diplomatic war of attrition against Gaddafi paying off in the end.

“We are dealing with a set of imperfect options,” a senior administration official said, noting that the measure of success is not “where things stand” but “where they would have stood had we done nothing.” The NATO airstrikes and a no-fly zone enforced by NATO and Arab countries “have essentially frozen the battle space in terms of the advance of Gaddafi’s forces,” he said, and “if you work all the other levers, you can make time work against Gaddafi.”

The official emphasized that Obama has no intention of sending U.S. ground forces — including noncombat military advisers — to Libya. But the administration’s attempts to firmly limit its involvement have also contributed to an image of disarray within NATO.

A senior European official, who spoke on the condition of anonymity to avoid antagonizing the Americans, said that Obama’s eagerness to turn over command of the Libyan air operation to NATO late last month, and the withdrawal of U.S. fighter planes from ground-strike missions, had undermined the strength of their united front against Gaddafi.

Although U.S. military officials have said that American strike aircraft remain “available” to NATO commanders should they request it, the senior administration official indicated that agreement would not be automatic.

“We would assess any requests,” the official said.

In a feisty response to any suggestion that the U.S. move to the back seat had undermined the NATO campaign, Vice President Biden said the alliance was perfectly capable of handling the air attack mission itself.

“It is bizarre to suggest that NATO and the rest of the world lacks the capacity to deal with Libya — it does not,” Biden said in an interview with the Financial Times.

“Occasionally other countries lack the will,” he said, “but this is not about capacity.”

Biden said U.S. resources were better spent trying to guide Egypt’s transition toward democracy. He denied that U.S. public reluctance to become deeply involved in another conflict in the Muslim world had anything to do with Obama’s decisions.

“This is about our strategic interest and it is not based upon a situation of what can the traffic bear politically at home,” Biden said. “The traffic can bear politically more in Libya,” he said, because “everybody knows [Gaddafi] is a bad guy.”

But as the situation in Libya has continued without resolution, popular disapproval of the president’s handling of the situation has shot up 15 percentage points, from 34 to 49, since mid-March, shining a light on the political risks Obama faces on the issue amid a host of domestic problems, according to a new Washington Post-ABC News poll. Among political independents, disapproval jumped to 51 percent.

Fifty-six percent of those polled said they agreed the United States should participate in the NATO-led international coalition; most of those said they wanted the level of U.S. participation to remain “about the same.” Most of those who supported participation appeared to agree with Biden’s assessment of Gaddafi, with 58 percent saying the goal of the operation ought to be getting rid of him.

France and Italy have recognized the opposition Transition National Council (TNC) as Libya’s legitimate government, and Britain, France and Italy have had civilian and military personnel on the ground at rebel headquarters in the eastern city of Benghazi for some time. But the new teams being sent were described as professionals capable of training rebel officers and organizing a more efficient command network for the ground war. Each country is expected to field at least 10 advisers.

France has also agreed to escalate the airstrikes being carried out by NATO. French President Nicolas Sarkozy met Libyan opposition leader Mustafa Abdel Jalil in Paris on Wednesday.

Although NATO is in command of the U.N.-authorized operation in Libya, different countries have agreed to different missions. Only six NATO members — France, Britain, Canada, Belgium, Denmark and Norway — are participating in the airstrikes against Libyan government forces on the ground. Others — along with non-NATO nations, including some Arab countries — are contributing aircraft to enforce a no-fly zone over Libya but have declined to participate in strikes. A third group is enforcing an arms embargo with air and sea patrols outside Libyan territory.

Those sending advisers or other assistance to the rebel forces are doing so as individual nations, in coordination with but outside the NATO command structure.

The three sending trainers have publicly ruled out sending ground forces. “The rebels themselves are afraid of being accused by other Arab countries of having allowed ‘crusaders’ on their land,” said an Italian official who was not authorized to discuss the issue on the record.

Qatar, one of several Arab countries whose planes are participating in the no-fly-zone enforcement, is also reportedly providing military equipment directly to the rebels, including weapons. “They’re sort of freelancing . . . throwing in money in a lot of different and odd ways,” including helping to finance a satellite television station to counter Gaddafi’s control of domestic media, said a senior congressional aide briefed on the operations.

Obama administration officials said their comfort level with the rebel council had grown in recent weeks, after high-level meetings with its leaders and direct contact by a U.S. diplomatic mission sent to Benghazi.

“Whether there are people in Libya who may have more extremist or nefarious agendas, that is something we watch very carefully,” the senior administration official said in reference to suggestions of possible al-Qaeda involvement. “We don’t believe that the organized opposition, represented by the TNC, reflects that agenda.”

Staff writer Mary Beth Sheridan contributed to this report.