Vice President Cheney was inside the main U.S. air base in Afghanistan yesterday when a suicide bomber detonated his explosives just outside the gates, killing as many as 23 people and showcasing insurgents' growing capabilities in advance of a widely expected spring offensive.

Within hours, a purported Taliban spokesman asserted responsibility for the attack -- which killed a U.S. soldier and an American civilian contractor -- and said it was an attempt to assassinate Cheney. U.S. officials disputed the assertion that Cheney was the target, noting that his overnight stay at the sprawling Bagram air base had been unplanned and that he was well away from the blast.

"I heard a loud boom," Cheney told reporters later. "The Secret Service came in and told me there had been an attack on the main gate."

The attack prompted military officials to issue a "red alert" at the base. Cheney was briefly moved to a bomb shelter, before being allowed to continue with his schedule.

Regardless of the intent, the attack demonstrated that insurgents in Afghanistan are becoming increasingly bold, willing to attack a heavily fortified U.S. target in the face of unusually tight security. Additionally, the assault was carried out in a part of the country where the Taliban has relatively little support. The Islamic militia's traditional stronghold has been in the south; Bagram is in the country's central region, about an hour's drive north of Kabul.

"It's pretty striking that they're capable of planning and executing an attack on Bagram on fairly short notice and under changing circumstances. We haven't seen anything like this before," said Daniel Markey, a senior fellow at the Council on Foreign Relations who until last month worked on South Asia policy at the State Department. "Psychologically, this has to be seen as a serious blow."

Markey said the attack is also an ominous sign with the approach of spring, which is usually accompanied by a heavy escalation in violence as conditions for fighting improve. "Everyone agrees on both sides that this is going to be a bad spring," he said.

Until 18 months ago, suicide bombings had been a rarity in Afghanistan, despite more than two decades of war. Recently, however, they have become a favored tactic of insurgents who are trying to undermine the weak pro-Western government in Kabul and force NATO troops to leave.

Last year, there were 139 suicide attacks in Afghanistan, five times as many as in 2005. The shift in tactics has prompted concern that the Taliban, which lost power after a U.S.-led invasion in 2001, is adopting the methods of insurgents in Iraq.

"This attack is a reflection of their new capabilities, which they've developed in the last year or two through their connections with the transnational extremists," said Ali Ahmad Jalali, former interior minister of Afghanistan. "This will boost morale and will help them to recruit more fighters. This is the kind of violence that can have a major psychological impact."

Asked whether Taliban fighters were sending a message, Cheney indicated they were. "I think they clearly try to find ways to question the authority of the central government," he said. "Striking at Bagram with a suicide bomber, I suppose, is one way to do that. But it shouldn't affect our behavior at all."

The Bush administration has become increasingly concerned by the violence in Afghanistan, and Cheney's previously unannounced stops in Pakistan and Afghanistan during a trip to Asia were intended to signal the White House's commitment to countering insurgents there.

Meeting with Pakistan's president, Gen. Pervez Musharraf, in Islamabad on Monday, Cheney pushed the leader to do more to crack down on Taliban and al-Qaeda activity in semiautonomous frontier region. Afghan officials have blamed Pakistan for allowing the Taliban a haven to plan and train for attacks in Afghanistan. U.S. officials had long defended Pakistan as a valuable ally, but intelligence reports that al-Qaeda is reconstituting itself in Pakistan have prompted U.S. officials in recent months to publicly question whether Musharraf is doing all he can.

Following the meeting with Musharraf, Cheney flew to Bagram late Monday. He stayed at the base overnight because a storm delayed his meeting in Kabul with Afghan President Hamid Karzai until yesterday morning. The delay was reported publicly Monday night.

Just before 10 a.m. yesterday, a suicide bomber driving a Toyota Corolla made it through a gate at the air base manned by Afghan police officers, according to Gen. Salem Ihsas, the police chief in Parwan province, where Bagram is located.

The assailant detonated his explosives before he was able to make it through a second gate, which is manned by U.S. personnel. Ihsas said that 19 people were killed in the blast and that 15 were wounded, mostly Afghan laborers who had reported for work at the base. Other estimates from Afghan officials put the death toll slightly higher, while U.S. authorities said nine people had been killed and 21 injured.

U.S. military officials said the bomber was on foot and did not get as close to the base as Ihsas suggested.

Ihsas said preliminary reports indicated the bomber was not Afghan but declined to say where he was from. "Once we are done with the investigation, we will know exactly," he said.

The dead in yesterday's blast included a South Korean soldier. South Korea is part of the 37-nation NATO-led International Security Assistance Force, which is responsible for security throughout Afghanistan.

The United States and Britain -- the two largest contributors of troops in Afghanistan -- both recently announced they would be increasing their force levels, with the U.S. total climbing to 27,000.

About 5,100 U.S. troops, plus 4,000 other coalition personnel and contractors, are stationed at the air base at Bagram.

Special correspondent Javed Hamdard in Kabul and staff researcher Robert E. Thomason in Washington contributed to this report.