The attacker struck a compound near the outer perimeter of Kabul airport, which was targeted last week when insurgents seized a building in the same area and fired towards the airport using automatic weapons and rocket-propelled grenades.
Taliban insurgents oppose the election process, which is currently undergoing an audit of all eight million votes due to a dispute between the two contenders over fraud allegations.
"The US Embassy strongly condemns the terrorist attack on Camp Gibson in Kabul this morning which killed five foreign national security guards," a US statement said.
It gave no further details on Camp Gibson, but Kabul police said it was a joint compound used by the interior ministry's counter-narcotics department and foreign forces.
"Our initial reports show the explosion took place inside the foreigners' compound," Kabul police chief Zahir Zahir told AFP. "The foreigners were exercising inside at the time."
"Our teams are on the ground investigating how an attacker on a motorcycle entered the compound."
The nationalities of the victims was not known, and there was no immediate comment from NATO's International Security Assistance Force (ISAF), which is winding down its mission after a 13-year war against the Taliban.
The Taliban used a recognised Twitter account to claim responsiblity for the attack, saying 15 "agents" had been killed inside a foreign intelligence base. The insurgents often exaggerate death tolls after attacks.
Afghanistan is on edge as the election dispute between poll rivals Ashraf Ghani and Abdullah Abdullah threatens to trigger instability and revive ethnic tensions that ravaged the country during the 1992-1996 civil war.
NATO combat troops will withdraw from Afghanistan by December, and reports suggest unrest is already worsening nationwide.
Civilian casualties soared by 24 percent in the first half of 2014, according to recent UN figures, while the International Crisis Group has said the "overall trend is one of escalating violence and insurgent attacks".
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