The violence came as U.S. military commanders warned in an interview with The Associated Press that the Taliban may try to subvert legislative elections in a week with "spectacular" assaults, using car bombs and suicide attackers.
But Maj. Gen. Jason Kamiya, the top operational commander in Afghanistan, said he was confident that enough American troops and other forces are in place to ensure the polls are a success.
Nine Afghan soldiers have been arrested in connection with the attempt to shoot Defense Minister Rahim Wardak, said ministry spokesman Gen. Mohammed Saher Azimi.
Four bullets hit his convoy as it was leaving the airport after dropping off him and several other ministers. One bullet hit "the exact place where the defense minister had been sitting in the car," Azimi said, and a defense ministry staffer was wounded.
"It is clear that it was an assassination attempt on the defense minister," he said.
The motive for the shooting was not immediately clear. A senior government official, speaking on condition of anonymity because he is not authorized to speak on the matter, said the soldiers were angry over a pay dispute.
The Sept. 18 elections are the next key step toward democracy after a quarter-century of war. The Taliban have stepped up attacks in the past six months, leaving more than 1,200 people dead, and have vowed to disrupt the polls.
A U.S. military intelligence official, Chief Warrant Officer Larry Tersone, told AP that the militants are expected to "start ramping up operations" toward voting day. He said the main threat was believed to be car bombs and suicide attacks against polling stations.
"I think they will try to conduct an operation of a spectacular nature within a significant population center because that is the immediate attention-getter they are looking for," he said.
He said the military was also on the lookout for lone militants using rockets against people queuing to vote and was prepared to deal with the threat.
But Kamiya said the military was prepared to deal with the threat.
"We are in a posture to disrupt, pre-empt and discourage enemy actions," he said.
Asked if the polls would be successful, Kamiya said, "I am 100 percent confident."
About 20,000 American troops are in Afghanistan as part of a 21,000-strong U.S.-led coalition. There is a separate force of 11,000 NATO-led peacekeepers.
Coalition forces have gone on the offensive in recent weeks, killing and arresting hundreds of suspected militants in raids across volatile southern and eastern regions in preparation for the polls.
In the latest operation, Azimi said Afghan and U.S.-led forces killed 30 suspected militants and arrested a large number of others Friday in southern Helmand province, which has suffered several insurgent attacks in recent weeks.
Scores of weapons were discovered in the area, including some that were lying in farm fields.
The helicopter crashed in Panjshir Valley, about 100 kilometers (60 miles) north of Kabul, as it was taking off, carrying army chief Bismillah Khan and three Cabinet ministers.
The chopper was about 30 meters (yards) in the air when it started to wobble, then quickly started descending, flipping on its side and crashed into the ground, said an Associated Press photographer at the scene. It burst into flames and then exploded.
Presidential spokesman Khaleeq Ahmed blamed the crash on the chopper's rotor blades clipping a tree during takeoff.
Hundreds of spectators, including government leaders and foreign ambassadors, fled screaming as chunks of metal slammed into a group of vehicles parked nearby.
The officials had attending a memorial in honor of Ahmed Shah Masood, the former head of the U.S.-backed Northern Alliance who was killed by two suspected al Qaeda assassins on Sept. 9. 2001.
Khan, Sediqa Balkhi, the minister for the disabled, and the chopper's two pilots were injured, Azimi said.
"The army chief of staff is in hospital but he is doing well," he said.
Separately, a helicopter carrying Afghanistan's army chief and three Cabinet ministers crashed and exploded in flames while taking off, but all on board managed to escape the burning wreckage with minor injuries. The government called it an accident.