A bomb hidden on a bus carrying Afghan laborers from a coalition base in southern Afghanistan exploded Thursday, killing 10 and wounding 15, police said.

The attack occurred as the bus was carrying the workers from the Kandahar Airfield, the coalition headquarters in southern Afghanistan, said Abdul Hakim Hungar, the deputy provincial police chief.

The blast destroyed the bus and left blood and body parts scattered on a road in downtown Kandahar city, a former Taliban stronghold.

There have been occasional attacks on Afghans working for coalition forces in the past, but this is one of the largest.

The attack occurred as coalition and national troops prepared to launch a sweeping anti-Taliban offensive, the biggest since the Islamic extremist government’s 2001 ouster.

Some 11,000 troops have deployed for the offensive in Helmand, one of four mountainous and desert-filled southern provinces being targeted. British, Canadian and Afghan troops are joining U.S. forces in the offensive.

On Wednesday, some 26 militants were slain in an attack on mountain positions in Paktika province, said provincial Gov. Akram Khelwak. Helicopter gunships and artillery fire supported ground troops; one Afghan police officer was wounded. Also in Paktika, four civilians died when rebel rockets slammed into their house, the provincial government said.

Meanwhile, in volatile Helmand province, U.S. troops in sweltering heat built sand barriers and guard outposts around a small forward operating base in support of Operation Mountain Thrust. Soldiers around the base’s perimeter fired rounds from 119-millimeter howitzers into the vast desert.

“We do it so they know it’s here and they know it could be pretty bad for them,” said Lt. Col. Chris Toner, commanding officer at the base in the Musa Qala district, 180 miles from the nearest permanent base, in Kandahar.

Afghanistan has been wracked by its bloodiest violence since the U.S.-led coalition invaded after the Sept. 11, 2001, terrorist attacks and toppled the Taliban government for harboring Osama bin Laden and his al-Qaida supporters.

Operation Mountain Thrust seeks to squeeze Taliban fighters responsible for a spate of ambushes and suicide attacks against coalition forces and Afghan authorities. It will focus on southern Uruzgan and northeastern Helmand, where the military says most of the militant forces have gathered. Operations will also be conducted in the former Taliban strongholds of Kandahar and Zabul.

“This is not just about killing or capturing extremists,” U.S. spokesman Col. Tom Collins said in Kabul, announcing the operation.

“We are going to go into these areas, take out the security threat and establish conditions where government forces, government institutions, humanitarian organizations can move into these areas and begin the real work that needs to be done,” he said, referring to reconstruction efforts.