Kabul suicide bombing kills 13 U.S. troops in deadliest bombing in months (VIDEO)

U.S. forces in Afghanistan have suffered their "deadliest insurgent attack in months," the Guardian reports, with 13 Americans killed in Kabul after a car filled with explosives rammed into the side of an armored bus transporting troops.

The International Security Assistance Force (ISAF) confirmed that five soldiers and eight civilian contractors were killed in the attack on the Rhino bus traveling  between NATO bases.

However, Afghan and western officials privately confirmed that all of the dead were from the U.S., making it the heaviest loss of American lives since a Chinook helicopter was shot down by the Taliban in August, killing 30 Americans and eight Afghans.

The size of the explosion reportedly made identification of victims difficult.

At least four Afghans were also killed, the LA Times reports.

The Taliban reportedly claimed responsibility for the bombing, using a four-wheel-drive vehicle packed with 700 pounds of explosives, in text messages to news organizations.

The vehicle was traveling from Camp Julien, which houses a counterinsurgency school for Afghan troops, to Camp Phoenix, which houses Americans who train  the Afghan army and police force.

The attack Saturday comes six weeks after insurgents laid siege to the U.S. Embassy in Kabul for 20 hours, killing more than a dozen people.

(GlobalPost's Jean MacKenzie reports from Kabul: Kabul attack: shape of things to come?)

Three Australian troops were also killed in the southern province of Uruzgan when an Afghan soldier opened fire.