Three suicide bombers rammed an explosives-laden truck into a construction depot in Eastern Afghanistan, killing 24 workers and wounding 59 others, officials said on Monday.
The bombers forced their way into the privately-run construction facility in Paktika province by shooting a security guard dead before blowing up their truck late Sunday, the interior ministry said.
The provincial administration condemned the attack in Barmal district, which shares a long, porous border with Pakistani areas troubled by Taliban militancy.
"At around 8.20pm, a truck filled with explosives entered the complex and detonated," it said in a statement. "Based on the information we have, 24 people were killed and 59 others were wounded."
Islamist insurgents in Afghanistan have been known to target construction workers — seen by them as in the pay of the Western-backed authorities in Kabul — as as well as military forces and government officials.
President Hamid Karzai said the attack was "unforgivable", calling it "the work of enemies of Afghanistan who oppose the country's development."
Taliban spokesman Zabihullah Mujahid said the group was responsible for the attack.
However, the Taliban, the main militant group behind the Afghan insurgency now into its 10th year, are known to make false claims and exaggerate casualties from their attacks.
The Al Qaeda-linked Haqqani network, which is based in Pakistan, is particularly active in Paktika and has been responsible for many attacks in the region.
It has also been blamed for some of the deadliest anti-US attacks in Afghanistan, including a suicide attack at a US base in 2009 that killed seven American CIA operatives.
In a separate incident on Sunday, five civilians were killed and two others wounded in an attack by unknown gunmen in the neighbouring province of Khost, authorities said. The motive for the killings was unknown.
Civilians are increasingly getting caught up in the violence that has blighted Afghanistan since a US-led invasion in 2001 ousted the Taliban, triggering an insurgency whose intensity has increased in 2011.
The United Nations says that last year was the deadliest for civilians since the conflict began, with 2,777 killed — a 15 per cent increase on 2009.
Three-quarters of the deaths were caused by attacks linked to the insurgents, with improvised bombs the biggest killers, taking the lives of 1,141 people.
Sunday's attacks underscore the huge security challenges Afghanistan faces, less than a week after President Hamid Karzai announced that local military and police will take over from Nato in seven parts of the country this summer.
The move, nearly 10 years after the US-led invasion in the wake of the September 11 attacks, is the first step towards the withdrawal of foreign troops from Afghanistan by the end of 2014.
Karzai said the Afghan people wanted to take charge of their own security, but he admitted that would not be an easy task for a country ground down by decades of war and still battling a brutal Taliban insurgency.