A suicide car bomb blew up Monday outside a US military base in eastern Afghanistan, killing nine civilian labourers, as the country marked Independence Day under the shadow of extremist attacks.
The blast, claimed by the insurgent Taliban, did not penetrate the base in the town of Khost, while security forces were able to prevent a second suicide attack moments later, the US-led coalition and Afghan officials said.
It came amid massive heightened security as Afghanistan marked Independence Day, commemorating its final defeat of the British army in 1919.
Kabul was locked down with 7,000 police on patrol and checkpoints at nearly every city centre intersection and main entry points into the capital.
President Hamid Karzai's annual Independence Day address in the stadium was called off with no explanation, while the defence ministry kept under wraps a smaller commemoration due later in the day.
A spokesman for the insurgent Taliban, Zabihullah Mujahed, said it carried out the suicide attack in Khost, 30 kilometres (19 miles) from the border with Pakistan.
The US-led coalition said insurgents detonated the device outside the base and that nine Afghans were killed and 13 wounded, according to first reports.
A secretary to the Khost governor, Mohammad Bilal, gave the same same toll. "It was a suicide car bombing against the gates of the Salerno camp," he said.
"Moments later a second car bomber came and wanted to detonate his bombs. Police identified him and opened fire on him," Bilal added.
He said the attacker was able to escape into the crowd and security forces destroyed the second bomb. "They wanted to disturb Independence Day."
The dead and wounded were labourers who had been waiting to enter the base for work, provincial government spokesman Khaibar Pashtun said.
Reacting to the suicide bombing, Karzai said in a statement that by killing "innocent civilians on Independence Day, the terrorists showed their hostility to the freedom of Afghan people."
The Al-Qaeda-linked Taliban were driven from power in a US-led invasion in late 2001.
However they were able to regroup, some of them taking refuge in Pakistan, to launch a snowballing insurgency which military officials say is attracting more Arab, Pakistani and other Muslim fighters.
The coalition and separate NATO-led International Security Assistance Force helping Afghanistan fight the extremists issued a rare warning of a "heightened security threat based on credible intelligence reporting."
"These reports indicate that the enemies of the people of Afghanistan intend to attack civilian, military and government targets during Afghan independence celebrations," a statement said.
UN staff were told to stay at home while other international personnel were told to restrict their movements.
"Staff are working at home as precaution," UN spokesman Aleem Siddique told AFP.
The capital has suffered a series of attacks in recent weeks.
The education minister escaped a roadside bomb Saturday, while two separate blasts in the city this month have killed two foreign soldiers and about seven Afghans.
The last major parade in the capital Kabul, on April 27, was disrupted when militants opened fire on a stage where Karzai, ministers, diplomats and other senior officials were seated.
Karzai survived but three people as well as three of the attackers -- said to be from the Taliban militia -- were killed.
In other violence reported Monday, a mine blew up a police vehicle in the province of Nangarhar and killed two policemen, an official said.
Meanwhile, the coalition said it had killed several militants on Sunday in separate operations in Paktia and Kapisa provinces. It did not give a number.