Afghan policemen patrol in front of a U.S. convoy near the site where a suicide bomber blew himself up in Bagram on Wednesday. (Ahmad Masood/Reuters)

Ten insurgents are dead and seven U.S. soldiers wounded following a brazen assault early Wednesday morning against the giant U.S.-run Bagram Airfield north of Kabul.

The attack began when suicide bombers blew themselves up at the front gate of the base at about 3:30 a.m. local time. Reports as to the number of bombers were unconfirmed but suggested there were four.

Insurgents also fired rockets, small arms and grenades into the base, one of the biggest military installations in Afghanistan.

U.S. forces used helicopter gunships to fly above the area and fire rockets into surrounding fields.

The gunfire lasted hours and finally subsided around midday, said Master Sgt. Tom Clementson, a spokesman for U.S. forces at Bagram.

The pre-dawn attack is the second Taliban strike at NATO forces in and around the capital in as many days.

On Tuesday, a suicide bomber struck a U.S. convoy in the capital of Kabul, killing 18 people including Canadian Forces member Col. Geoff Parker and five American troops. It was the deadliest attack on NATO in the Afghan capital in eight months.

The back-to-back attacks appeared part of a Taliban offensive that the insurgents announced earlier this month — even as the U.S. and its partners prepare for a major operation to restore order in the turbulent south.

Taliban claim responsibility

The Taliban claimed responsibility for both the Kabul bombing and the attack at Bagram, about 50 kilometres north of Kabul. Taliban spokesman Zabiullah Mujahid said 20 suicide attackers were involved.

A building was damaged in the attack but it was "not a mission-essential building," said Maj. Virginia McCabe, a Bagram spokeswoman. While they could still hear occasional small-arms fire around the base late in the morning, "it is becoming more and more sporadic," she said.

NATO's senior civilian representative in Afghanistan said insurgents were prevented from breaching the base's defences.

"We know that a group of insurgents sought to penetrate the air base and that's been dealt with," Mark Sedwill told reporters.

Soldiers, insurgents engage in gunfight

An Afghan provincial police commander, Gen. Abdul Rahman Sayedkhail, said the attack began when U.S. guards spotted would-be attackers in a car just outside the Bagram base. The Americans opened fire, triggering a gunfight in which at least one militant triggered his suicide vest. A gunfight broke out as U.S. troops hunted down the other attackers.

The Bagram assault occurred following the deadliest day of the year for U.S. forces in Afghanistan with seven Americans dead — including two who died in separate attacks in the south. Twelve Afghan civilians also died in Tuesday's blast — many of them on a public bus in rush-hour traffic along a major thoroughfare that runs by the ruins of a one-time royal palace and government ministries.

The attacks followed a Taliban announcement earlier this month of a spring offensive — "Operation Al-Fatah" or "Victory" — which would target NATO forces, foreign diplomats, contractors and Afghan government officials.

In February 2007, a suicide bombing killed more than 20 people at a Bagram security gate while former vice-president Dick Cheney was inside the base. Cheney was unhurt but the Taliban said he was the target.

With files from the CBC's Darrow McIntyre