The Taliban attack in Kabul on Saturday that killed at least 12 Americans, a Canadian and four Afghans highlights the insurgents’ growing reliance on high-profile bombings in the capital and targeted assassinations that seem designed to destroy Afghans’ confidence in their struggling government.

A vehicle laden with explosives swerved into an armored U.S. military bus, resulting in one of the deadliest strikes aimed at Americans in Kabul in the past decade, according to U.S. military and Afghan officials.

The American and Canadian dead included five soldiers and eight civilian contractors. The attack was the latest in a series of spectacular and frequently suicidal assaults in major cities against government and military targets.

In recent weeks, Taliban fighters waged a prolonged gun-and-grenade battle aimed at the U.S. Embassy in Kabul and killed a key Afghan peace envoy and former president in a suicide bomb attack.

The shift in Taliban strategy has been driven, in part, by the addition of 30,000 U.S. troops who have pushed insurgent fighters out of their rural havens in the south and made it harder for them to attack front-line U.S. combat forces.

In the wake of Saturday’s attack, U.S. commanders sought to highlight their gains over the past year. Gen. John R. Allen, the top U.S. commander in Afghanistan, said the bombing was designed “to hide the fact that [the Taliban] are losing territory, support and the will to fight.”

But the attacks in the previously safe capital also highlight the Taliban’s resilience at a time when the United States is beginning a gradual drawdown of its forces in the country and trying to press forward with stalled peace talks.

U.S. officials in recent months have held preliminary talks with the Taliban and the affiliated Haqqani network, both of which operate out of sanctuaries in Pakistan.

The American strategy envisions continued military pressure combined with a sustained push to jump-start reconciliation talks and grow Afghanistan’s army and police force. Secretary of State Hillary Rodham Clinton last week described the approach as “fight, talk, build.”

Saturday’s suicide bombing is likely to bolster critics who have insisted that the prospects of reconciliation with the Taliban remain remote. In testimony Thursday, Clinton acknowledged meeting with a representative of the Haqqani network, which has been behind most of the high-profile Kabul attacks and has links to Pakistan’s intelligence service.

Asked how the network had responded, she said that the answer was “an attack on our embassy.” A week later, a suicide bomber killed former president Burhanuddin Rabbani, the head of Afghanistan’s High Peace Council.

A senior administration official, who spoke on the condition of anonymity because he was not authorized to speak on the record, said it is unrealistic to expect that the Taliban would scale back attacks in advance of serious peace negotiations. “What do you expect?” the official said. “They are in a war. We are aggressively trying to kill Taliban and Haqqani [fighters], and they are trying to kill us.”

Frequent convoys in capital

Until recently, violence in Kabul, particularly against U.S. troops, had been rare. American forces do not regularly patrol in the capital, leaving the job to Afghan soldiers and police officers. U.S. soldiers and civilians, however, frequently move between their headquarters and other bases in Kabul.

The suicide bombing Saturday targeted one such convoy, exploding against a large armored bus as it passed the private American University. Shrapnel sprayed across a four-lane highway and reduced the armored bus to smoking wreckage.

The Taliban asserted responsibility for the bombing, claiming that 1,500 pounds of explosives had been used.

The U.S. death toll was the highest in a single incident since Aug. 6, when insurgents shot down a Chinook helicopter in Wardak province, killing 30 U.S. Special Operations troops and eight Afghans.

The most recent suicide attack took place not far from Darulaman palace, the bombed-out former kings’ residence that sits on a hill on the city’s western outskirts. The highway is routinely traveled by NATO convoys, and insurgents have targeted them before, as in a May 2010 bombing that killed 18 people, including five U.S. troops.

After the Saturday blast, U.S. and Afghan troops blocked off the road as they cleaned up the wreckage and ferried casualties to hospitals. At least four Afghans died, including one policeman, the Interior Ministry said in a statement. A hospital official in Kabul said eight other people were wounded.

The Associated Press reported that a Canadian soldier was among the five troops killed.

Also Saturday, a man in an Afghan army uniform opened fire at a NATO-Afghan base in the southern province of Uruzgan, killing three Australian troops.

New limits on Americans

The recent Kabul attacks and the deaths of as many as eight U.S. civilians in Saturday’s bombing are likely to lead to new restrictions on the ability of American officials to move around the capital. In contrast to Baghdad, U.S. troops and civilians had been able to move around the city with ease and relatively little security.

The tighter restrictions could make it harder for U.S. officials to mentor an Afghan government that is already plagued by incompetence and corruption.

Kabul is one of seven areas where the United States has transitioned the lead responsibility for security to Afghan army and police forces. As U.S. forces continue to be withdrawn, the number of areas under the control of Afghan security forces will grow. Afghan President Hamid Karzai is expected to announce in Istanbul next week that Afghan forces are prepared to take security control in all or parts of 17 of Afghanistan’s 34 provinces.

U.S. officials brushed off suggestions that attacks such as Saturday’s suicide bombing could slow the move to hand over security responsibility to the Afghans. “Transition is not about peace, harmony and stability,” the senior administration official said. “It is getting to a point where the local security forces can handle the threat with support.”

Jaffe reported from Washington. Staff writer Karen DeYoung in Washington contributed to this report.