KHOST, Afghanistan - Three suicide bombers killed 20 people in an attack on a construction firm in a restive province in southeastern Afghanistan, government officials said on Monday, with the Taliban claiming responsibility for the assault.

Violence in Afghanistan has spiraled in the past year, with Taliban-led militants stepping up their fight against the Afghan government as Kabul prepares to take over responsibility for security gradually from foreign forces.

The attackers forced their way into the company's compound in the Bermel district of volatile Paktika province on Sunday night after killing a security guard, and then detonated a truck laden with explosives, the Interior Ministry said.

"As a result, 20 employees of the construction company were killed and 50 others were injured," it said in a statement.

Mohebullah Sameem, governor of Paktika, earlier put the death toll from the attack at 13.

He said the dead and wounded included employees of the firm and other civilians. Construction crews and others working on infrastructure projects are frequently targeted by insurgents.

Mukhlis Afghan, governor Sameem's spokesman, said the firm was one of the largest construction companies funded by the US government in the province.

He said the blasts were so powerful that doors and windows were blown out of nearby houses, but he did not know if there were casualties outside the firm's compound.

Bermel shares a long border with neighboring Pakistan, where insurgents are said to have safe havens from which they launch attacks inside Afghanistan.

On Sunday, the NATO-led International Security Assistance Force said it had killed more than 10 insurgents in an air strike in the same district.

In an e-mailed statement to media, Taliban spokesman Zabihullah Mujahid claimed the Islamist group had carried out the attack but said it had been on a military base and that 49 foreign and Afghan troops had been killed and wounded.

Taliban insurgents often inflate casualties inflicted on Afghan government forces and foreign troops.

Violence across Afghanistan last year reached its worst levels since the Taliban were ousted by US-backed Afghan forces in 2001, with civilian and military casualties hitting record levels.

The violence underscores the challenges ahead as US and NATO forces begin to hand over security responsibility to Afghan troops, allowing foreign troops to withdraw gradually from an increasingly unpopular war.

The process, announced last week, will begin with the handover of seven areas in July and culminate in the withdrawal of all foreign combat troops by 2014.

Reuters