ASHINGTON, March 28 — Federal authorities will order major security improvements at several of the nation's largest airports after inspections showed that passenger planes taking off or landing at those airports would be vulnerable to attack by terrorists using shoulder-fired missiles, senior Bush administration officials said.

The inspections, which began several weeks ago, are being conducted by a federal task force created by the White House late last year after terrorists linked to Al Qaeda tried to shoot down an Israeli passenger plane on takeoff from an airport in Kenya in November. The two small, shoulder-fired missiles barely missed the plane.

Administration officials would not identify the airports that would be required to make major safety improvements, citing security reasons. But they said the list included several of the nation's busiest, and that the improvements would include new, round-the-clock security patrols and tightened electronic surveillance of the flight paths used for takeoffs and landings.

This week, dozens of National Guard troops were deployed to the Los Angeles International Airport to patrol the perimeter and road checkpoints, in part because of what security officials acknowledged was concern about shoulder-fired missiles.

A spokesman for the Port Authority of New York and New Jersey, which manages Kennedy, La Guardia and Newark Liberty International airports, said it was aware of the missile threat and was responding to it. The spokesman, Pasquale DiFulco, said the port authority had a policy of not discussing details of its security planning. "But we have certainly taken the necessary steps and precautions to address these issues," Mr. DiFulco said.

Bush administration officials said that nationwide inspections, which have been carried out at roughly 80 airports by officials of the Federal Bureau of Investigation, the Department of Homeland Security and other federal agencies, demonstrated that a terrorist with a shoulder-fired antiaircraft missile weighing as little 30 pounds would find it relatively easy to evade security at many large airports and fire a missile that could bring down a passenger plane.

American intelligence and law enforcement agencies say that Al Qaeda and affiliated terrorist groups are believed to have an arsenal overseas of dozens of shoulder-fired missiles, including the American-made Stinger and the Russian SA-7, and that others can be bought by terrorists on the black market for several thousand dollars each.

Many of the Stingers available on the black market are left over from the American-backed guerrilla effort in Afghanistan to force out Soviet troops in the 1980's. The Stinger and missiles like it are capable of shooting down planes several miles away at heights of more than 10,000 feet.

Administration officials stressed that they had no evidence to suggest that Al Qaeda or other terrorist groups had managed to smuggle any of the small missiles into the United States, or that they intended to try.

"We are not aware of any credible, specific intelligence information that Manpad attacks are being planned against commercial aircraft in the U.S. at this time," said James M. Loy, director of the Transportation Security Administration, using the acronym for Man Portable Air Defense Systems, the technical name for the missiles. "The administration does, however, recognize the potential threat."

Officials say the attempt to shoot down the Israeli plane in Kenya last November had created alarm in Washington that Al Qaeda would try a similar attack in the United States. The incident near the international airport at Mombasa came six months after a similar Russian-made missile was fired at an American military plane in Saudi Arabia. That missile also missed.

American intelligence officials say that in the attacks in both Kenya and Saudi Arabia, the planes may have been saved by antimissile technology that is routinely installed in Israeli passenger jets and United States military planes.

There is no similar federal requirement that antimissile defense systems be built into American passenger planes. But since the Kenya attack, a growing bipartisan movement in Congress has called for the installation of antimissile systems on American-owned commercial planes in the United States.

After a Congressional briefing on the issue by representatives of the Central Intelligence Agency and the Pentagon earlier this month, Representative John L. Mica, a Florida Republican who is chairman of the House Transportation Subcommittee on Aviation, said that the missile threat was sobering and "we can't afford not to act." Mr. Mica said that he would seek immediate federal financing to protect commercial planes against terrorist missiles.

Federal aviation officials say that it costs about $1 million to $2 million to outfit a passenger plane with equipment to deflect a missile.

There are a variety of different types of antimissile technologies available, including a system that releases decoy flares to draw a heat-seeking missile away from a plane; other systems use jamming equipment to interfere with a missile's guidance system.

Lawmakers say that among the American passenger planes likely to be outfitted first would be those used routinely for flights overseas, especially to parts of the world where Al Qaeda and other terrorist groups are known to exist.

In a speech last month to an air travel conference in Thailand, Steven J. McHale, the No. 2 official in the Transportation Security Administration, warned his Asian counterparts about the dangers of portable missiles and called on their governments to step up security around their airports and to make sure that their own military stocks of the missiles were secure.

"There are thousands available on the gray and black markets, and many of these are finding their way into the hands of terrorist groups," Mr. McHale said of the portable missiles. He warned that a single missile attack on a passenger plane in Asia "would likely cripple international aviation."

Asa Hutchinson, the under secretary for border and transportation security in the Department of Homeland Security, said in an interview that the federal government had taken a variety of steps in recent months to deal with the missile threat at major airports, although he could not reveal details for security reasons. "We're much better off than we were some time ago," Mr. Hutchinson said. "But certainly this is a threat and a vulnerability that we're concerned with."

While the federal government has the authority to order airports to step up counterterrorism measures, Mr. Hutchinson said the Department of Homeland Security understood the budget constraints on many large airports. "There will be negotiation," he said. "Obviously we're not going to be telling them to do something they're totally incapable of doing."

The worldwide inventory of portable surface-to-air missiles is estimated at 700,000, according to government officials and private weapons specialists. The vast majority of those missiles are in government arsenals, but munitions specialists say that shoulder-fired missiles like the Stinger and the SA-7 are so small and portable that many are diverted from government stocks each year and sold on the black market. A Stinger is about 5 feet long, 6 inches wide and weighs about 35 pounds when fully armed.

John Iannarelli, a spokesman for the F.B.I., said that many airports were being asked to step up their training of security officers to be on the lookout for the missiles. "A lot of this is educational," Mr. Iannarelli said. "These are rather conspicuous weapons, and we want to make sure that it is recognized for what it is."

Administration officials said some large airports had stepped up anti-missile security measures long before the federal inspectors arrived.

Logan Airport in Boston is flanked on three sides by water, and airport officials have provided local clam diggers with mobile phones to allow them to call in if they see suspicious activities.

The chief spokesman at San Francisco International Airport, Mike McCarron, said that inspectors from the Transportation Security Administration completed their review of the airport's antimissile defense this month. Mr. McCarron said airport officials believed San Francisco International was better protected than others because it is surrounded by water, allowing the Coast Guard to keep a close eye on activities nearby. He said that the airport had also stepped up its road patrols on the perimeter of the airport.

"We think we've got a pretty good idea of what's going on," he said.