WASHINGTON, Nov. 5 - American intelligence agencies have tripled their formal estimate of shoulder-fired surface-to-air missile systems believed to be at large worldwide, since determining that at least 4,000 of the weapons in Iraq's prewar arsenals cannot be accounted for, government officials said Friday.

A new government estimate says a total of 6,000 of the weapons may be outside the control of any government, up from a previous estimate of 2,000, American officials said.

The officials said they did not know whether missiles from Iraq remain there or have been smuggled into other countries, though a senior administration official said Friday that "there is no evidence that they have left the country.''

It was unclear whether Iraqi military or intelligence personnel removed the missile systems during the initial invasion of Iraq or whether they disappeared from warehouses after major combat ended.

Shoulder-fired missiles - which are small, lethal and easy to use - are attractive weapons for terrorists. In recent months, Western intelligence and law enforcement agencies have repeatedly warned that Al Qaeda intends to use them to shoot down planes. In 2002, attackers who launched two small Russian-made SA-7 missiles almost hit a commercial aircraft taking off from Mombasa, Kenya. The new estimate of a larger number of the missile systems was discussed at a classified Defense Intelligence Agency conference in Alabama this week, the officials said. They declined to discuss the methods by which the new estimate had been reached, saying that it was classified.

American intelligence analysts have said in the past that during Saddam Hussein's rule, Iraq stockpiled at least 5,000 of these missile systems, and that fewer than a third had been recovered. The shelf life of the missiles can vary, with battery life depending on the conditions under which they are stored.

Secretary of State Colin L. Powell said last fall that "no threat is more serious to aviation" than the shoulder-fired missiles, which can be bought on the black market for as little as $5,000, are about five feet long and weigh as little as 35 pounds. More than 40 aircraft have been struck by shoulder-fired surface-to-air missiles since the 1970's, causing at least 24 crashes and more than 600 deaths worldwide, according to a State Department estimate. In Iraq, the missiles have been used in more than a dozen attacks on American planes and helicopters, including those taking off and landing at Baghdad's international airport.

In recent months, the number of successful missile attacks on American aircraft and helicopters in Iraq has declined, but American officials have said the reason has largely been the precautionary measures taken by the United States military.

An unclassified study released in June 2004 by what is now the Government Accountability Office cited "U.S. government estimates" that a few thousand of the portable missiles were "outside government controls.'' A separate study released in November 2003 by the Congressional Research Service cited counterterrorism experts in saying that as many as 4,000 to 5,000 shoulder-fired missiles might be available to Iraqi insurgents.

The new estimate by American intelligence agencies was described by government officials who had access to the classified intelligence report. They said the tripling of the number represented the first formal effort to determine how unaccounted Iraqi stockpiles may have compounded the surface-to-air missile threat. Only several hundred shoulder-fired surface-to-air missiles from the Iraqi arsenals have been turned in to American forces in a buyout program, the government officials said.

A Defense Department official said Friday that more than one million shoulder-fired surface-to-air missiles had been produced since the weapons were first manufactured in the 1950's, with 20 countries producing more than 35 different types of weapons. According to the accountability office study, 500,000 to 750,000 shoulder-fired surface-to-air missiles are still believed to be in the worldwide inventory. Many of the older missiles are militarily obsolete and have been destroyed.

Until the invasion of Iraq, many of the shoulder-fired weapons believed to be outside government controls were those provided by the United States and its allies to mujahedeen fighters in Afghanistan to assist in their resistance against Soviet forces during the 1980's. Those weapons included American-made Stinger and British-made Blowpipe missiles, but by December 2002, American-led forces in Afghanistan had captured more than 5,000 of the missiles from the Taliban and Al Qaeda, according to news reports at the time.

The Defense Intelligence Agency conference on the worldwide threat to civil aviation posed by these portable air defense systems was held Wednesday and Thursday by the agency's Missile and Space Intelligence Center, at Redstone Arsenal, in Huntsville, Ala.

The range and accuracy of the weapons can vary widely by type, with the Russian-made SA-16 regarded as the most lethal in Iraq's prewar arsenal. It is not known how many of the missiles may have been fired at American planes and helicopters during the invasion in 2003.

In an effort to address the missile threat, the Department of Homeland Security has asked government contractors to find a way to protect passenger jets from small shoulder-fired missiles. The technology has been installed on military planes for years, using laser-jamming equipment and decoy flares to deflect the missiles, and some contractors have determined that passenger planes could be outfitted with antimissile technology relatively soon.

The State Department has also started an aggressive effort to persuade other countries to join in an effort to limit the availability and proliferation of the weapons. In July, the House of Representatives passed a bill that calls on the president to pursue even stronger measures, and directs the administration to expedite approval of new antimissile technologies. The Senate has not yet acted on the bill.

The new government estimate follows the disclosure late last month that more than 300 tons of powerful explosives had disappeared since early March 2003 from an Iraqi site previously monitored by the International Atomic Energy Agency. Video images that emerged after that report appears to suggest that at least some of the explosive material disappeared after the fall of the Iraqi government. Unlike those explosives, surface-to-air missiles in Iraq were not sealed or monitored by weapons inspectors before the war and may have been widely dispersed among the Iraqi forces in the field.

Commentary:
Who's in charge of Iraqi weapons? George W. Bush. Are you ready for impeachment yet?