KABUL, Afghanistan - A Taliban suicide bomber rammed a vehicle loaded with explosives into an armored NATO bus Saturday on a busy thoroughfare in Kabul, killing 17 people, including a dozen Americans, in the deadliest strike against the U.S.-led coalition in the Afghan capital since the war began.

The blast occurred on the same day that a man wearing an Afghan army uniform killed three Australian soldiers and an Afghan interpreter in the south - attacks that show the resiliency of the insurgency and are likely to raise new doubts about the unpopular 10-year-old war and the Western strategy of trying to talk peace with the Taliban.

A spokesman for the fundamentalist Islamic movement, which was ousted in the 2001 invasion for its affiliation with al-Qaida, claimed responsibility for the Kabul attack, saying the bomber had used 1,540 pounds of explosives.

The Taliban and related groups have staged more than a dozen major attacks in Kabul this year, including seven since June, in an apparent campaign to weaken confidence in the Afghan government as it prepares to take over its own security ahead of a 2014 deadline for the U.S. and other NATO countries to withdraw their troops or move them into support roles.

Underscoring the difficulties ahead, the brazen assault occurred just hours after top Afghan and Western officials met in the heart of Kabul to discuss the second phase of shifting security responsibilities to Afghan forces in all or part of 17 of the country's 34 provinces. Afghans already have the lead in the Afghan capital.

Heavy black smoke poured from the burning wreckage of an armored personnel carrier, known as a Rhino, in Kabul after the bomber struck. The bus had been sandwiched in the middle of a convoy of mine-resistant military vehicles when it was hit along a four-lane highway often used by foreign military trainers in the southwestern part of Kabul.

The landmark Darulaman Palace, the bombed-out seat of former Afghan kings, was the backdrop to the chaotic scene: Shrapnel, twisted pieces of metal and charred human remains littered the street.

U.S. soldiers wept as they pulled bodies from the debris, said Noor Ahmad, a witness at the scene. One coalition soldier was choking inside the burned bus, he said.

"The bottom half of his body was burned," Ahmad said.

NATO said five of its service members and eight civilian contractors working for the coalition died in the attack.

A U.S. defense official, speaking on condition of anonymity to release the information before a formal announcement, said all 13 were Americans. However, Lt. Col. Christian Lemay, a Canadian defense spokesman, told The Associated Press that one Canadian soldier was among the troops killed. The discrepancy could not immediately be reconciled.

It was the deadliest single attack against the U.S.-led coalition across the country since the Taliban shot down a NATO helicopter on Aug. 6 in an eastern Afghan province, killing 30 U.S. troops, most elite Navy SEALs, and eight Afghans.

The Afghan Ministry of Interior said four Afghans, including two children, also died in Saturday's attack. Eight other Afghans, including two children, were wounded, said Kabir Amiri, head of Kabul hospitals.

In all, there were three attacks Saturday against NATO and Afghan forces across the country.

A teenage girl also blew herself up as she tried to attack an Afghan intelligence office in the capital of Kunar province, a hotbed of militancy in northeast Afghanistan along the Pakistan border, the coalition said. Abdul Sabor Allayar, deputy provincial police chief, said the guards outside the government's intelligence office in Asad Abad became suspicious and started shooting, at which point the bomber detonated her explosives, killing herself and wounding several intelligence employees.

Afghan Defense Ministry spokesman Mohammad Zahir Azimi said officials were investigating whether the man who opened fire on a joint NATO-Afghan base in the restive southern Uruzgan province was an actual soldier or a militant in disguise.

The Australian Broadcasting Corp. said the attack occurred during a morning parade at a forward patrol base in southern Kandahar province, and the gunman wearing an Afghan army uniform was later killed. The discrepancy in the location of the attack could not immediately be clarified.

In Canberra, the Defense Department said three Australlian soldiers and their Afghan interpreter were killed in the attack, and seven Australian soldiers were wounded.

"It's a huge loss," said U.S. Ambassador Ryan Crocker. "Our deepest sympathies go out to their comrades and families, but it will not deter us from our mission. It's a shock, but we will not let these guys win."

Just a day earlier, the Pentagon issued a progress report saying that the number of enemy-initiated attacks in Afghanistan was trending downward. Since May of this year, the monthly number of these attacks has been lower than the same month in 2010, something not seen since 2007, it said.

However, the Pentagon also noted that the insurgency's safe havens in Pakistan and the limited capacity of the Afghan government could jeopardize efforts to turn security gains on the battlefield, primarily in the south, into long-term stability in Afghanistan.

Saturday's attack broke a relative lull in the Afghan capital, which has experienced a number of attacks in recent years that are often blamed on the Haqqani network, an al-Qaida and Taliban-linked movement that operates out of Pakistan.

The most recent attack in Kabul was the Sept. 20 assassination of former Afghan President Burhanuddin Rabbani by an insurgent who detonated a bomb hidden in his turban. The attacker was posing as a peace emissary coming to meet Rabbani, who was leading a government effort to broker peace with the Taliban.

That occurred about a week after teams of insurgents firing rocket-propelled grenades and automatic weapons struck at the U.S. Embassy, NATO headquarters and other buildings in the heart of Afghanistan's capital, leaving seven Afghans dead.

On Saturday, NATO and Afghan forces sealed off the blast area as fire trucks and ambulances, sirens blaring, rushed in. Coalition troops carried a badly burned body on a stretcher and several black body bags to two NATO helicopters that landed nearby to airlift casualties from the scene.

The Taliban identified the bomber as Abdul Rahman and said he was driving a Toyota Land Cruiser SUV containing 1,540 pounds (700 kilograms) of explosives and targeting foreigners providing training for Afghan police. The Taliban, who frequently exaggerate casualty claims, said that 25 people were killed by the blast.

A similar attack occurred on the same road in May 2010 when a suicide bomber struck a NATO convoy, killing 18 people. Among the dead were five U.S. soldiers and a Canadian colonel.