BAGRAM, Afghanistan — In what the Taliban claimed was an assassination attempt, a suicide bomber attacked the main gate of a US military base within earshot of Vice President Dick Cheney. The explosion killed 23 people, including two Americans, and delivered a propaganda blow that undercut the US military and the weak Afghan government it supports.The bomber struck about 10 a.m. yesterday, and US military officials declared a “red alert” at the sprawling Bagram Air Base while Cheney was rushed to a bomb shelter. Cheney, who had been stranded at the base overnight by a snowstorm, met with President Hamid Karzai in the capital before heading back to the United States via the Gulf state of Oman.

“I heard a loud boom,” Cheney told reporters aboard Air Force Two en route to Oman. “The Secret Service came in and told me there had been an attack on the main gate.”

Many of the victims were said to be Afghan truck drivers waiting to get inside the base. A dozen men — many of them sobbing heavily — left the base holding a stretcher bearing their loved ones wrapped in black body bags. Tears streamed down the face of one man sitting in the passenger seat of an SUV that carried another victim away.

Although the bomber did not get closer than roughly one mile to the vice president, the attack highlighted an increasingly precarious security situation posed by the resurgent Taliban. Five years after US-led forces toppled their regime, Taliban-led militants have stepped up attacks. There were 139 suicide bombings last year, a fivefold increase over 2005, and a fresh wave of violence is expected this spring.

Asked if the Taliban were trying to send a message with the attack, Cheney said: “I think they clearly try to find ways to question the authority of the central government. Striking at Bagram with a suicide bomber, I suppose, is one way to do that. But it shouldn’t affect our behaviour at all.”

Cheney was the highest-ranking US official to stay overnight in either the Afghanistan or Iraq war zones.

A message posted on a website used by militants said “a mujahid (holy warrior) ... carried out a suicide attack in front of the second gate of the Bagram Air Base. ... The target was Bush’s vice president, Dick Cheney.”

A purported Taliban spokesman, Qari Yousef Ahmadi, said Cheney was the target of the attack carried out by an Afghan named Mullah Abdul Rahim.

“We knew that Dick Cheney would be staying inside the base,” Ahmadi told The Associated Press by telephone from an undisclosed location. “The attacker was trying to reach Cheney.”

But the bomber never tried to get by any US-manned security checkpoints and instead walked into a group of Afghans outside the base and detonated himself, said US spokesman Lt. Col. David Accetta.

“To characterise this as a direct attempt on the life of the vice president is absurd,” Accetta said.

Cheney’s trip to Afghanistan had not been announced in advance.

Even though reaching the front gate of the US base could have been achieved with relative ease, the idea of getting through US security to attack Cheney was “far-fetched,” in the words of Maj. William Mitchell, a US spokesman.

Nevertheless, Seth Jones, an Afghan expert at the RAND Corp. think tank, said the attack was a “stark reminder of the deteriorating security environment” in Afghanistan and was a propaganda boost for the Taliban.

Karzai’s office said 23 people were killed, including 20 Afghan workers waiting outside the base. Twenty other people were injured, it said.

NATO said nine people had been killed, including a US soldier, a US contractor and a South Korean soldier, but Col. Tom Collins, the top spokesman for the NATO force, said the numbers from Karzai’s office were probably “close to the truth.”

During their private hour-long meeting, Cheney and Karzai spoke about the “problems coming from Pakistan,” said an Afghan government official, a reference to cross-border infiltration by militants who launch attacks in Afghanistan.