A van laden with explosives swerved into a U.S. military armored bus Saturday, killing at least 12 Americans and at least four Afghans in one of the deadliest days of the decade-long war for Americans in the Afghan capital, according to U.S. military and Afghan officials.

The NATO-led International Security Assistance Force said the dead included five service members with the coalition and eight civilian employees.

A Canadian defense spokesman said that a Canadian soldier was among the five troops killed, the Associated Press reported.

The Taliban attack struck at a central goal of the U.S. military here: protecting Kabul and other major cities from the type of high-visibility violence that leads Afghans to fear that their government cannot protect them. U.S. troops deployed as far away as the Pakistani border fight in the farmlands and mountains to disrupt insurgents intent on bringing their bombs to Kabul.

Such violence is rare in the city, particularly against U.S. troops, who do not regularly patrol there; they leave that to Afghan soldiers and police, although they do move between their headquarters and other bases in Kabul. The van targeted one such convoy Saturday, exploding against a large armored vehicle known as a Rhino as it passed the private American University, reducing the troop carrier to smoking wreckage and spraying shrapnel across a four-lane highway.

The Taliban asserted responsibility for the bombing, claiming that 1,500 pounds of explosives had been used. In the past, attacks in Kabul have often been the work of the Haqqani network, an affiliated group of insurgents based in Pakistan’s tribal areas who have links to Pakistan’s intelligence service.

The attack was further evidence of an evolution in Taliban tactics. The insurgency has proved less capable this year of confronting the bulk of U.S. troops based in southern and eastern Afghanistan; violence levels this fighting season have fallen. But it has marshaled its resources for high-profile assaults in major cities against government and military targets, as well as for assassinations of top Afghan officials.

Last month, a team of Taliban fighters waged a prolonged gun-and-grenade battle against the U.S. Embassy in Kabul from a building overlooking the downtown compound. Seven Afghans were killed in that attack, although there were no U.S. casualties. Also last month, a suicide bomber killed an Afghan peace envoy and former president, disrupting the nascent efforts to negotiate with the Taliban. In August, insurgents stormed a British cultural center in the capital, leaving eight people dead.

Saturday’s bombing resulted in the highest U.S. death toll 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 attack took place not far from Darulaman palace, the bombed-out former kings’ residence that sits on a hill on the western outskirts of the city. The area is one of the most common places in Kabul to see NATO convoys as they travel between nearby bases, and insurgents have targeted them before, as in a May 2010 bombing that killed 18 people, including five U.S. troops.

The Rhino is a boxy armored bus with small bulletproof windows that was regularly used by U.S. troops in Iraq to navigate the dangerous road from the Baghdad airport into the city.

After the blast, U.S. and Afghan troops blocked off the road and kept bystanders back 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 at least eight other people were wounded. One witness, Ahmed Sakhi, was riding his bicycle nearby when the massive explosion occurred, and he said he saw the bodies of women and children lying in the road.

The precarious security in Kabul and elsewhere in Afghanistan is looming larger now that the U.S. military is planning to start withdrawing troops later this year and the Afghan government is rolling out its second phase of “transition,” selecting areas where its forces will begin to take over more responsibility from the U.S.-led NATO coalition. The Taliban ranks have taken heavy losses in recent years but have proved their resiliency time and again, with their leadership protected across the border in Pakistan.

Also Saturday, a man in an Afghan army uniform opened fire at a NATO-Afghan base in the southern province of Uruzgan, killing three coalition troops whom news reports identified as Australian. There have been several other incidents in which Afghan security forces, or insurgents disguised as them, turned their weapons on their partners.

A teenage girl also blew herself up at an Afghan intelligence agency building in Konar province, along the Pakistan border in the east, wounding several intelligence officials.

Summing up the day’s violence, Gen. John Allen, the top U.S. commander in Afghanistan, said in a statement that he was both “saddened and outraged by the attacks.”

“The enemies of peace are not martyrs, but murderers,” he said. “To hide the fact that they are losing territory, support, and the will to fight, our common enemy continues to employ suicide attackers to kill innocent Afghan fathers, mothers, sons and daughters, as well as the Coalition forces who have volunteered to protect them.”

Pentagon spokesman George Little said that Defense Secretary Leon Panetta had been told of the convoy attack and that “his heart goes out to those who were killed and wounded, and to their families.”

Salahuddin is a special correspondent. Special correspondent Javed Hamdard in Kabul and staff writers Craig Whitlock and Greg Jaffe in Washington contributed to this report.