NATO forces in Kandahar were trying Sunday to figure out how a small band of insurgents was able to mount an audacious attack on the biggest coalition base in southern Afghanistan, firing a barrage of mortars and rockets at Kandahar Airfield and mounting a ground assault on the base.

Taliban insurgents fired at least five rockets and mortars at the massive airbase, NATO's main base of operations in southern Afghanistan, and tried to breach the perimeter shortly after nightfall Saturday.

On Sunday night, a U.K. military blog quoted a source which said that Canadians were among the more than 12 foreign troops injured during the attack.

According to The Canadian Press, the Helmand Blog spoke with Senior Aircraftsman Eric Telford from the Royal Air Force. One of the injured Canadians is a female, the blog reported.

Meanwhile, CP reporter Tara Brautigam told Â鶹´«Ã½ Channel that in the wake of the attack, security around the base had been "visibly stepped up."

"There are more people wearing flak jackets, more military personnel walking around scouring the base," he said on Sunday.

Brautigam said the entire base echoed with the sounds of the firefight, including artillery rounds, machine-gun and automatic rifle fire, which ebbed and flowed several times during the attack.

"NATO forces responded and drove away or repelled the attack," he said.

But the International Security Assistance Force (ISAF) is saying little else about the nighttime attack by Taliban gunmen that left several civilians and military personnel injured.

Squadron Leader Paul Scott, a spokesperson for the NATO-led force, said officials were still investigating the attack and could not say how many insurgents were involved, whether any escaped after the four-hour assault, or how many casualties there were on either side.

He said the insurgents failed to get into the base during their attack, which was aimed at the northern edge of the sprawling Kandahar base, and no one was killed. However, some civilians and members of NATO military forces were wounded.

The ISAF has not released the number of injured or their nationalities.

The Canadian Forces said the military does not release information about its wounded troops.

The Associated Press reported that one of the rockets appeared to have struck a shop-lined boardwalk where soldiers often spend their off-duty hours. A bloodstain could be seen on the walkway outside one restaurant on the boardwalk Sunday morning, said Maura Axelrod, a reporter with HDNet who was inside the base.

Insurgent attacks not unusual

It is not unusual for the Taliban to launch sporadic rocket attacks on the sprawling base, which is built around an airstrip a half an hour's drive south of Kandahar City.

Most of these attacks have come in the form of Chinese-made rockets, propped up against a pile of rocks, crudely aimed at the NATO base and fired remotely. They usually do little damage, even when they hit the base.

But a ground attack is extremely rare, particularly on such a heavily fortified base. The Taliban has largely avoided engaging in head-to-head combat with coalition forces since 2006, when a series of operations by Canadian soldiers culminating in Operation Medusa killed hundreds of Taliban fighters in Kandahar province.

Taliban spokesperson Qari Yousef Armada told The Associated Press that its fighters attacked the base from two sides and fired more than 15 rockets.

It was the third major attack on NATO forces in Afghanistan in six days.

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

On Tuesday, a Taliban suicide bomber attacked a NATO convoy in the capital, killing 18 people, including five American and Col. Geoff Parker, the highest-ranking Canadian soldier to die in the Afghan mission to date.

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