KANDAHAR, Afghanistan - Maj. Robert Kelly had just finished dinner and was walking back to his shack at Kandahar Airfield late Saturday when he heard two explosions nearby, the start of an attack on the base.

The 35-year-old St. John's, N.L., native immediately hit the gravel and heard a rumble.

"From the explosions I could hear overhead -- it was probably fragmentation from whatever the initial rockets hit," said Kelly, an operations officer for the Canadian military's National Support Element.

"It was shrapnel, bits of building, I'm not quite sure what it was. But you could hear like three pieces fly over and it's as close as I've heard them fly over. It was loud enough to make you really duck down and say, 'Whoa, that was pretty close."'

In an audacious four-hour assault, insurgents fired rockets and mortars into the airbase in a failed attempt to breach the northern perimeter of NATO's largest military installation in southern Afghanistan.

The nighttime attack at the heavily fortified base occurred at about 8 p.m. local time. A small number of military personnel and civilians were injured and treated.

The nationalities of the wounded were not released. The Canadian Forces did not say whether any Canadians were injured in the attack, as it no longer releases that information. There were no confirmed fatalities.

Troops with the International Security Assistance Force, including Canadian soldiers, donned flak jackets, grabbed their weapons and patrolled the base until the assault gradually ended.

At least five rockets and mortars were fired at the vast Kandahar airbase. But no insurgents entered the base after they were repelled by security forces, ISAF said in a statement.

As soon as the rockets hit the base, troops and civilians hit the ground before scurrying for cover in concrete bunkers.

Three hours into the attack, the boom of artillery and the rattle of gunfire could be heard in the distance. Sirens blared intermittently as the attack was underway.

Everyone on the base was instructed to remain in safe locations as a precaution until the attack concluded at about midnight.

Prime Minister Stephen Harper and Defence Minister Peter MacKay were both briefed about the attack.

"Obviously it reminds us that our men and women in uniform in all capacities over there, including on the airfield, do face constant danger," Harper told reporters during an appearance at an event in Dryden, Ont.

Isolated rocket attacks are commonplace at the base, but they are often wildly inaccurate. Ground attacks such as the one Saturday night, however, are rare.

"This was kind of different," Kelly said.

"Not that we weren't expecting this because there's been activities now over the last five or six days with Bagram and the convoy in Kabul. But KAF is such a large base that there's 25,000, 30,000 people here now ... it didn't seem like it was overly co-ordinated."

It's the third major assault on NATO's military hubs in Afghanistan in six days.

On Tuesday, a Taliban suicide bomber attacked a NATO convoy in Kabul, killing 18 people including Col. Geoff Parker, the highest-ranking Canadian soldier to die in the Afghan mission to date. Five U.S. servicemen also died.

The following day, dozens of Taliban militants attacked the main U.S. military base -- Bagram Airfield -- killing an American contractor in fighting that lasted more than eight hours. Nine others were wounded.

The sprawling Kandahar base, long the Canadian military's main base in Afghanistan, has grown to the size of a small city. It has become the launching pad for additional U.S. forces pouring into the country for a summer surge against the Taliban.

The attack Saturday came two weeks after the Taliban announced a spring offensive against NATO forces and Afghan government troops -- their response to a promise by U.S. President Barack Obama to flush the Taliban out of their strongholds in southern Kandahar province.

Attacks in the south earlier Saturday killed three NATO service members and a civilian working with the military, NATO said in a statement. It did not provide further details.