By Matt Kelley, Associated Press

WASHINGTON � The United States has yet to catch Osama bin Laden and other senior al-Qaida leaders, and the terrorist network responsible for the Sept. 11 attacks remains a threat despite 20,000 American bombs dropped on Afghanistan in a war now costing $2 billion a month.

The U.S.-led fight against terrorism has driven the Taliban from power in Afghanistan and destroyed al-Qaida's training camps and main hiding places there. Still, experts say more attacks can be expected.

Stability has yet to come to Afghanistan, as shown Thursday by an assassination attempt on interim leader Hamid Karzai and a deadly car bombing in Kabul. There is the continuing threat of a biological, chemical or nuclear terrorist attack -- often cited by Defense Secretary Donald H. Rumsfeld and other Bush administration officials.

Experts are questioning how well the campaign is proceeding, 11 months into a military action that has cost more than $15 billion and taken the lives of 39 U.S. soldiers and hundreds of Afghan civilians.

"I think we have actually done a lot less than we assumed six or seven months ago," said Ivo Daalder, a national security aide in the Clinton administration and now at the Brookings Institution.

"It's a mixed bag so far. It's becoming more and more of a negative bag because we seem to have stalled for the past six months in our operations there in Afghanistan and Pakistan."

Al-Qaida has been hurt -- "We've given them a few body blows," said terrorism expert Warren Bass -- thought to what extent is unclear.

"We really have surprisingly little information other than the fact that our main goal -- to deny Afghanistan as a sanctuary for al-Qaida -- has been achieved," said Anthony Cordesman, a terrorism expert at the Center for Strategic and International Studies.

"We have no intelligence reporting on how thoroughly al-Qaida has been hit," he said. Also unknown is precisely what has been achieved with the law enforcement, financial, economic and political weapons that President Bush has brought to bear.

Bush administration officials say they do not know where bin Laden is, or if he was killed in battle or by his reported kidney problems.

Many senior Taliban and al-Qaida leaders are at large. The United States did capture Abu Zubaydah, the group's former operations chief, and No. 3 leader Mohammed Atef died in an airstrike.

Many analysts say the failure to get bin Laden is a major shortcoming.

"He's a six-foot Arab on dialysis. You wonder how long he can wander around unnoticed," said Bass, director of the terrorism program of the Council on Foreign Relations.

"Other terrorist organizations have really gone downhill after losing their leader ... Bin Laden has charisma and money and a brand name in a way that his replacement might not."

Others disagree.

"I think it's too high a standard to say you'll capture him, particularly given the way we've fought the war: fairly remotely and through surrogates," said military analyst Michael Vickers of the Center for Strategic and Budgetary Assessments.

Bush and his Cabinet members say the goal of the fight against terrorism is not to capture or kill any one person but to destroy terrorist networks' ability to strike. They say routing the Taliban and al-Qaida from Afghanistan has been essential to that goal.

Airstrikes in Afghanistan began Oct. 7. By December the United States had helped Afghan rebel forces oust the Taliban and destroy most of al-Qaida's infrastructure in the country.

U.S. and coalition airplanes have conducted more than 21,000 flights over Afghanistan, dropping more than 20,000 munitions. About 60 percent of the ordnance dropped on Afghanistan has been precision guided, the highest percentage in any conflict. The Pentagon says the war is now costing about $2 billion a month, compared with $1.2 billion earlier this year.

Seventy countries have joined the anti-terror fight, with 24 besides the United States sending forces to Afghanistan.

The United States has sent some of its newest weaponry into battle in Afghanistan, including bombs that use an explosive mist to blow out caves and pilotless Predator drones rigged by the CIA to shoot missiles.

Thirty-nine Americans have died in the war, including 16 during combat or other hostile situations. The largest number came March 4, when seven U.S. soldiers were killed during Operation Anaconda, the biggest air and ground assault of the war.

Four Canadian soldiers and three U.S. soldiers have been killed by mistaken or errant U.S. airstrikes. The bombing is believed to have killed hundreds of Afghan civilians, too, including about 50 in a July 1 strike on central Afghan villages.

Military officials say the U.S. presence in Afghanistan probably will continue for years as soldiers search for remaining Taliban and al-Qaida and help train a new Afghan national army to take over from warlords' militias.

The United States has avoided joining the International Security Assistance Force providing security in Kabul and has rejected calls for U.S. troops to help expand the peacekeeping force outside the Afghan capital.

On Thursday, Deputy Defense Secretary Paul Wolfowitz said expanding an international peacekeeping force would benefit Afghanistan, but other countries would have to do it.

Critics say that is another shortcoming of the war so far.

"It's no good having a foreign policy approach balanced between 'We don't want the world's help' and 'We can't do this, the world is going to have to clean this up,"' Bass said.

Speaking shortly before news broke of the assassination attempt on Karzai and the Kabul car bomb, Wolfowitz said Afghanistan was a safer place than a year ago.

"The security situation in Afghanistan is not collapsing," Wolfowitz said in a speech at the Brookings Institution.

But experts say al-Qaida is a danger and there seems little doubt that terrorism and the war against it are far from over.

"For many people, what has really not sunk in is this is not going to go away," Cordesman said. "For the rest of their lives, these uncertainties are going to be a fact."