Less than a month into the Libyan conflict, NATO is running short of precision bombs, highlighting the limitations of Britain, France and other European countries in sustaining even a relatively small military action over an extended period of time, according to senior NATO and U.S. officials.

The shortage of European munitions, along with the limited number of aircraft available, has raised doubts among some officials about whether the United States can continue to avoid returning to the air campaign if Libyan leader Moammar Gaddafi hangs on to power for several more months.

U.S. strike aircraft that participated in the early stage of the operation, before the United States relinquished command to NATO and assumed what President Obama called a “supporting” role, have remained in the theater “on 12-hour standby” with crews “constantly briefed on the current situation,” a NATO official said.

So far, the NATO commander has not requested their deployment. Several U.S. military officials said they anticipated being called back into the fight, although a senior administration official said he expected other countries to announce “in the next few days” that they would contribute aircraft equipped with the laser-guided munitions.

Opposition spokesmen in the western Libyan city of Misurata, under steady bombardment by government shelling, said Friday that Gaddafi’s forces had used cluster bombs, and Human Rights Watch said its representatives on the ground had witnessed the explosion of cluster munitions in civilian areas there. The Libyan government denied the weapons had been used.

A spokesman for the Misurata City Council appealed for NATO to send ground troops to secure the port that is the besieged city’s only remaining humanitarian lifeline.

The opposition has also repeatedly called for an increase in NATO airstrikes. The six countries conducting the air attacks, led by Britain and France, were unsuccessful at a meeting this week in Berlin in persuading more alliance members to join them.

NATO officials said that their operational tempo has not decreased since the United States relinquished command of the Libya operation and withdrew its strike aircraft at the beginning of April. More planes, they said, would not necessarily result immediately in more strike missions.

But, they said, the current bombing rate by the participating nations is not sustainable. “The reason we need more capability isn’t because we aren’t hitting what we see — it’s so that we can sustain the ability to do so. One problem is flight time, the other is munitions,” said another official, one of several who were not authorized to discuss the issue on the record.

European arsenals of laser-guided bombs, the NATO weapon of choice in the Libyan campaign, have been quickly depleted, officials said. Although the United States has significant stockpiles, its munitions do not fit on the British- and French-made planes that have flown the bulk of the missions.

Britain and France have each contributed about 20 strike aircraft to the campaign. Belgium, Norway, Denmark and Canada have each contributed six — all of them U.S.-manufactured and compatible with U.S. weaponry.

Since the end of March, more than 800 strike missions have been flown, with U.S. aircraft conducting only three, targeting static Libyan air defense installations. The United States still conducts about 25 percent of the overall sorties over Libya, largely intelligence, jamming and refueling missions.

Other NATO countries, along with the United Arab Emirates, Qatar and Jordan, have contributed planes to enforcing a no-fly zone over Libya to prevent Gaddafi’s use of airpower, but so far have declined to participate in the strike missions.

After the Berlin meeting, NATO Secretary General Anders Fogh Rassmussen said that 10 more aircraft were needed and that he was confident they would be supplied. A U.S. official said that Italy — which earlier in the week said it was not interested — may contribute planes to the ground attack mission, and that the Arab participants might also do so.

But with Gaddafi’s forces and the rebel army locked in a stalemate, Obama has resisted calls from opposition leaders, and some hardline lawmakers in this country, to move U.S. warplanes back into a leading role.

Sen. John McCain (R-Ariz.) and other have called on Obama to redeploy U.S. AC-130 gunships, which are considered more effective over populated areas.

Although the gunships flew several missions early in the operation, Gen. Carter Ham, who commanded the mission before it was turned over to NATO, said last week that they were frequently grounded because of weather and other concerns.

The slow-moving aircraft, which flew as low as 4,000 feet over Libya, are also considerably more vulnerable than jet fighters to surface-to-air missiles. While much of Libya’s stationary air defenses have been destroyed, Ham said Gaddafi was believed to have about 20,000 shoulder-held SAMS at the beginning of the conflict, and “most” of them are still unaccounted for.

Concerns that supplies of jet-launched precision bombs are growing short in Europe have reignited long-standing controversies over both burden-sharing and compatibility within NATO. While allied jets have largely followed the U.S. lead and converted to precision munitions over the last decade, they have struggled to keep pace, according to senior U.S. military officials.

Libya “has not been a very big war. If [the Europeans] would run out of these munitions this early in such a small operation, you have to wonder what kind of war they were planning on fighting,” said John Pike, director of GlobalSecurity.org, a defense think tank. “Maybe they were just planning on using their air force for air shows.”

Despite U.S. badgering, European allies have been slow in some cases to modify their planes and other weapons systems so they can accommodate U.S. bombs. Retooling these fighter jets so that they are compatible with U.S. systems requires money, and all European militaries have faced significant cuts in recent years.

Typically, the British and French militaries buy munitions in batches and stockpile them. When arsenals start to run low, factories must be retooled and production lines restarted to replace the diminished stock, all of which can take time and additional money, said Elizabeth Quintana, an aerospace analyst at the Royal United Service Institute in London.

Correspondent Simon Denyer in Tripoli contributed to this report.