KABUL, Afghanistan - Insurgents firing rockets, mortars, and automatic weapons launched a ground assault Saturday against NATO's biggest base in southern Afghanistan, wounding several coalition troops and civilian employees in the second such attack on a major military installation in the last week, officials said.

A Canadian Press news agency report from the Kandahar Air Field said that artillery and machine-gun fire reverberated through the base, about 300 miles southwest of Kabul, several hours after the attack began. Militants unleashed rockets and mortars about 8 p.m. local time and then tried unsuccessfully to storm the northern perimeter, officials said.

No one immediately claimed responsibility for the assault - the third major attack on NATO forces in Afghanistan in six days - but the Kandahar area is a Taliban stronghold.

On Tuesday, a Taliban suicide bomber attacked a NATO convoy in the capital, killing 18 people, 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.

Navy Cmdr. Amanda Peterseim, a spokeswoman for NATO forces at Kandahar, said that at least five rockets struck the base in the initial attack, but witnesses said explosions continued through much of the night. There were no reports of deaths, and she did not have the precise number of wounded.

"The alarm has been sounding for several hours, but no insurgents have penetrated the base perimeter," NATO said in a statement. It said "a number" of military and civilian personnel were wounded "and are receiving medical treatment. There are no confirmed fatalities."

NATO said troops and civilians were told to remain in bunkers as a precaution.

Peterseim 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 south of Kandahar city, are not uncommon. But ground assaults against such large facilities as Kandahar and Bagram are rare. Two attacks in the same week show that the militants are capable of complex operations despite NATO military pressure.

The attacks follow a Taliban announcement of a spring offensive against NATO forces and Afghan government troops - their response to a promise by the Obama administration to squeeze the Taliban out of its strongholds in southern Kandahar province.