KABUL (AFP) ― A Taliban suicide attacker riding a motorbike killed at least four foreigners in a compound in Kabul on Tuesday, police said, in the latest blast to rock the capital during an impasse over presidential election results.

The attack hit close to the outer perimeter of Kabul airport, which was targeted last week when insurgents seized a building in the same area and fired toward the airport using automatic weapons and rocket-propelled grenades.

The Taliban 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.

“Our initial reports show the explosion took place inside the foreigners’ compound ― four foreigners were killed and six were wounded,” Kabul police chief Zahir Zahir said. “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 and jobs of the victims was not known, and there was no immediate comment from NATO’s International Security Assistance Force, which is winding down its mission after a 13-year war against the Taliban.

“The attack happened inside a joint compound used by the counter narcotics department (of the Interior Ministry) and foreign forces,” Hashmat Stanakzai, spokesman for Kabul police said.

“Also at around 7 a.m., another bomb exploded (in the same district) slightly wounding one civilian.”

Another government official said that the victims worked for a private contractor.

The Taliban used a recognized 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 U.N. figures, while the International Crisis Group has said the “overall trend is one of escalating violence and insurgent attacks.”

The vote audit was part of a deal brokered by U.S. Secretary of State John Kerry to end the political crisis as Ghani and Abdullah both claimed victory.

Some 23,000 ballot boxes are being transported by the Afghan army and NATO forces to the capital, where they will be examined at 100 verification stations.

Abdullah led Ghani after the first round of voting but preliminary results of the runoff, announced on July 7, showed Ghani ahead by over one million votes.

Abdullah rejected the result, saying that most of his opponent’s ballots were fraudulent.

Abdullah draws his support from Tajiks and other northern Afghan groups, while Ghani is backed by Pashtun tribes of the south and east.

The recount has already sparked fresh arguments and fallen behind schedule.

The inauguration of President Hamid Karzai’s successor was due on Aug. 2, but has now been delayed.

Kabul airport, which includes a large military base as well as a civilian terminal has often been attacked before, including a rocket strike that destroyed Karzai’s parked helicopter on July 3.