Nine civilians have been killed and 13 others injured in a suicide bomb attack in southeastern Afghanistan.
Officials said a suicide bomber rammed an explosives-packed car into the gate of the main US military base. The Taliban claimed responsibility for the blast.
The bombing comes at a time of heightened security as military and diplomatic officials issued warnings that intelligence reports indicate the al-Qaeda-backed Taliban is planning a large attack to coincide with today's Independence Day celebrations.
Two more suicide bombers in another car approached the base near the southeastern town of Khost as security forces dealt with the aftermath of the first attack.
They were shot dead by police before they were able to detonate their explosives, Khost's governor, Arsala Jamal, told wire services.
'The victims were all poor labourers and civilians. This was a barbaric act carried out by the enemies of Afghanistan at a time of celebration of independence,' he said.
The explosion did not penetrate the perimeter of the base, the US military said, and US forces evacuated the wounded to the hospital at the camp, the main hub for operations in southeastern Afghanistan.
Insurgent attacks
Violence has surged in Afghanistan this year as the Taliban steps up its campaign of guerrilla attacks, backed by roadside and suicide bombs to oust the pro-Western Afghan government and drive out foreign troops.
The commander of US-led coalition forces in Afghanistan issued a warning that credible intelligence indicated a possible attack on Independence Day.
'These reports indicate that the enemies of the people of Afghanistan intend to attack civilian, military and government targets during Afghan Independence celebrations,' the US military said in a statement.
'We recommend to all Afghans to be vigilant at large public events and other locations where crowds gather and report suspicious behaviour to security forces, Afghan or coalition, and government representatives,' it said.
The Interior Ministry said more than 7,000 police have been deployed in Kabul for the Independence Day celebrations, which will be held in a secret location after President Hamid Karzai survived a Taliban assassination bid during a parade in April.
The ministry said the deployment of extra personnel was the biggest operation of its kind in Kabul since 2001, when US-led forces ousted the Taliban government.
UN staff in the capital were ordered to work from home, a spokesman said.
Meanwhile, an investigation is under way after four civilians were apparently killed in the country's Helmand province in a rocket attack by British troops.
Afghanistan is celebrating full independence from Britain, attained in 1919. Though never part of the British Empire, Britain had controlled Afghan foreign policy since the second Anglo-Afghan war of 1879.
