U.S. spokeswoman Capt. Elizabeth Mathias says the blast outside the main base at Bagram did not appear to kill anyone.
Sgt. Joel Peavy, a spokesmen at Bagram, told CBS News Fazul Rahim that a car parked outside the main gate to the base blew up, and minutes later a man running away from the scene detonated explosives attached to his body, injuring three U.S. civilian contractors. He did not have any further details on the victims' identities.
Peavy told CBS News there were no U.S. military or ISAF (International Security Assistance Force) casualties.
The governor of Afghanistan's Parwan province, where Bagram is located, about 47 miles north of Kabul, confirmed to Rahim that there was a vehicle bomb, followed by a suicide bomber detonating his explosives as he ran toward the gates. Gov. Abdul Jababr Taqwa said guards had identified the man as a bomber and he was shot before he blew himself up.
The attack came a day after three ISAF soldiers were killed in a blast at another base in southern Afghanistan, according to a statement released by the coalition on Wednesday.
"It is with grief that we inform you of the loss of these brave soldiers as we offer our heartfelt condolences to their families," said ISAF spokesman Brigadier-General Richard Blanchette, according to the statement.
The coalition did not release the identities or nationalities of the troops killed, or the exact location of the base hit by the "improvised explosive device."
Meanwhile, Afghanistan's election commission announced Wednesday that the presidential election should be held Aug. 20, rejecting a request from President Hamid Karzai.
The commission said the election couldn't be held earlier because of security issues and logistical problems.
Karzai last week asked the commission to see if the vote could be held in line with the Afghan constitution. The charter says Karzai must step down on May 22 and that elections must be held 30 to 60 days before that.