Insurgents launched a rare ground assault against NATO's main military base in southern Afghanistan on Saturday, wounding several U.S. service members in the second such attack on a major military installation this week, officials said.

No one immediately claimed responsibility for the attack _ the third major assault on NATO forces in Afghanistan in six days.

On Tuesday, a Taliban suicide bomber attacked a NATO convoy in the capital, killing 18 people including six NATO service members including five Americans and a Canadian.

The next day, dozens of Taliban militants attacked the main U.S. military base _ Bagram Air Field _ killing an American contractor in fighting that lasted more than eight hours.

Rockets started hitting Kandahar Air Field about 8 p.m. local time (15:30 GMT), followed quickly by a ground assault, said Navy Commander Amanda Peperseim, a spokeswoman for NATO forces at the base. She said the attack was still ongoing and did not provide further details.

She said at least five rockets struck the base, wounding a number of service members, as militants tried unsuccessfully to breach the defense perimeter on the northern side. There were no reports of deaths and she did not have the precise number of wounded.

Peperseim did not know how many insurgents launched the attack but said they did not appear to be wearing suicide vests, as had many of those who stormed the Bagram Air Field north of Kabul on Wednesday. In addition to the U.S. contractor's death, 16 militants were killed and five attackers were captured in the Bagram assault.

Rocket attacks against the Kandahar base, located about 10 miles (16 kilometers) south of Kandahar city, are not uncommon. But ground assaults against such large facilities as Kandahar and Bagram are rare, and two attacks in the same week show that the militants are capable of complex operations despite NATO military pressure.

The attacks came soon after the Taliban announced a spring offensive against NATO forces and Afghan government troops _ their respone to a promise by the Obama administration to squeeze the Taliban out of their strongholds in southern Kandahar province.

Kandahar Air Field is the launching pad for thousands of additional U.S. forces pouring into the country for a summer surge against the Taliban.

Attacks in the south earlier Saturday killed three NATO service members _ one American, one French and one Dutch _ and an Afghan interpreter. That brought to 996 the number of U.S. service members who have died since the war began in October 2001.

A loudspeaker announcement at the Kandahar base said the ground attack was coming from the north, said Maura Axelrod, a reporter with HDNet who was inside the base. She said she could hear heavy outgoing fire and that commanders had come into the bunker where she had taken cover to order all Marines with weapons to help in establishing a security perimeter.

An Afghan named Najibullah who works with a private security company on the base said that he heard rockets hitting for about half an hour. He only gave one name.

NATO's current push is aimed at winning over the population in Taliban-friendly areas by establishing security and bolstering the local government. However, each military strike has created potential for backlash amid arguments about who is truly an insurgent.

In the latest such incident, at least a dozen people were killed south of the capital Saturday after U.S. troops spotted two insurgents trying to plant bombs, an Afghan official said.

The two were shot dead in Paktia province, district chief Gulab Shah said. Troops saw comrades drag the two bodies away and called in a helicopter gunship which killed 10 more people, whom U.S. officials said were all militants, Shah said.

Shah said Afghan authorities will investigate to make sure the dead were all insurgents.

Civilian deaths are a flashpoint issue in Afghanistan, where President Hamid Karzai has urged NATO to take all necessary measures to protect civilian lives.

More than eight years into the war in Afghanistan, international support is also weakening.

The defense minister of Britain's new Conservative-Liberal Democrat coalition government met with Karzai in Kabul on Saturday and said he hopes to speed the withdrawal of British troops.

Defense Secretary Liam Fox is quoted in Saturday's edition of The Times newspaper he "would like the forces to come back as soon as possible," and wants to see if it is possible to speed the training of Afghan troops.

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Associated Press Writers Mirwais Khan in Kandahar and Mike Corder in The Hague, Netherlands, contributed to this report.