ISLAMABAD - Gunmen attacked an army jeep in Pakistan's capital Thursday, killing a brigadier on leave from a UN peacekeeping mission and his driver in the second deadly attack to strike Islamabad in 48 hours.

The brazen shooting in a smart residential district capped a spike in attacks blamed on Taliban and Al-Qaeda-linked extremists leaving more than 185 people dead this month in the frontline state in the US-led war on terror.

The assassination of a senior officer came less than two weeks after militants staged an audacious ambush on the nearby army headquarters in Rawalpindi claimed by the Tehreek-e-Taliban in Pakistan (TTP) movement.

Pakistan has vowed to crush the network, unleashing a major ground and air offensive targeting TTP in their South Waziristan stronghold along the Afghan border where Al-Qaeda are accused of plotting attacks on the West.

The attackers fled on a motorbike after shooting Brigadier Moinuddin Ahmed in the G11 sector, a hospital official said.

"There were two people riding a motorbike. One opened fire on the jeep from the front side and one from the back," senior police official Syed Kalim Imam told reporters.

"The brigadier succumbed to his injuries and his driver was also killed."

A hail of gunfire left the front and back windows of the green jeep riddled with bullet holes, spraying glass into the road.

Doctor Minhajus Siraj, deputy executive director of Islamabad's main PIMS Hospital, said Ahmed was shot in the face and chest and his driver killed.

A soldier injured in the attack, who had been in the brigadier's jeep, was stable after surgery.

A military official said Ahmed served for the United Nations in Sudan, where Pakistan contributes personnel to the UN peacekeeping forces deployed after a two-decade civil war between north and south, and in conflict-ridden Darfur.

There was no immediate claim of responsibility but Pakistan's security establishment have been increasingly targeted by Islamists.

Pakistan is on heightened alert for attacks with the military pressing a major offensive against TTP's estimated 10,000 fighters in South Waziristan, part of a lawless tribal belt.

Prime Minister Yousuf Raza Gilani condemned Thursday's shooting, the eighth militant attack this month.

"When you hit their (Taliban) nerve centres they will retaliate," security expert Saad Muhammad Khan told AFP.

"It is very serious that they have selected their target right inside the capital. They did it two days ago and they did it again today.

"We must be one step ahead of the terrorists but the situation is the other way round. They are one step ahead."

Millions of students have been kept at home this week as Pakistan shut all schools and colleges after a suicide attack Tuesday at a university.

In a sign of growing global alarm, international tennis organisers cancelled two junior tournaments in Pakistan, both due to begin in the next fortnight.

Officials say at least 137 militants and 18 soldiers have been killed since the South Waziristan offensive began Saturday and more than 120,000 civilians have fled the war zone.

A vanguard of troops have established footholds and are thrusting into the second phase of the operation, backed by helicopter gunships and warplanes, to home in on Taliban strongholds, a military official told AFP Thursday.

There was heavy fighting on the outskirts of Kotkai, the hometown of TTP chief Hakimullah Mehsud and between Taliban-stronghold Sararogha and Jandola, home of a large military base, where the army said 13 militants were killed.

Washington has encouraged the offensive, but officials said US Congress may approve tough new restrictions on military aid to Pakistan, which has decried similar curbs on assistance as undermining its sovereignty.