A suicide car bomber rammed a US consulate vehicle in Pakistan on Monday, killing at least two people in the deadliest attack targeting Americans in the country in more than two years.

Up to 19 people were wounded when the bomber struck during morning rush hour in the northwestern city of Peshawar near the office of the UN Refugee Agency (UNHCR) and residential quarters used by the US consulate.

There was no immediate claim of responsibility, but it was at least the third time since April 2010 that the consulate and its staff have been targeted by militants linked to the Taliban and Al-Qaeda.

US Secretary of State Hillary Clinton condemned the "cowardly" attack in the frontline state of the war on Al-Qaeda and praised the response of the local authorities, while refusing to speculate who might be behind the blast.

There was some confusion over the death toll -- two bodies were recovered from the scene but several Pakistani sources including a provincial minister insisted four people died, including two Americans.

The US embassy issued an initial statement quoting State Department spokeswoman Victoria Nuland as saying "no US Consulate personnel" were killed, though two Americans and two Pakistanis working for the mission were wounded.

A spokeswoman later said that the US embassy was "not aware" that any American citizens were killed.

But Mian Iftikhar Hussain, information minister for Pakistan's Khyber Pakhtunkhwa province, said four people were killed in the attack on the two-vehicle convoy, two of them Americans.

"When the bombing occurred, people sitting in the rear vehicle removed the people from the front vehicle and took them back towards the American Club within a minute," he said. The American Club is a high-security guest house run by the US consulate for its staff.

A half-burnt US passport was recovered from one of the vehicles damaged in the blast, and local police station chief Shahjahan Afridi said investigators have found one of the suicide bomber's legs.

He said one of the bodies found at the scene has been identified as a local man while the other was burned beyond recognition.

"I have no information about foreigners because details have not been shared with us," he said.

Peshawar police chief Imtiaz Altaf said 19 people were wounded in the blast and that the bomber's vehicle had been packed with up to 110 kilos (240 pounds) of explosives, including more than 10 mortar shells.

Witness Mohammad Sadid said he was driving to work at his pharmacy when he heard a deafening blast on the opposite side of the road in University Town neighbourhood.

"It was so powerful that it jolted my car with a massive jerk. My head banged the steering wheel and the windshield. I couldn't understand what had happened. I saw a car on fire. It became a large fireball," he said.

Taliban and Al-Qaeda-linked militant groups who are sworn enemies of the United States frequently carry out attacks and have strongholds in the tribal belt near Peshawar.

In May 2011, a Taliban bomb damaged a US consulate vehicle and wounded two US government employees in Peshawar, just days after American commandos killed Osama bin Laden in a raid on his Pakistan hideout.

In April 2010, a Taliban gun, grenade and double suicide car bomb attack on the consulate killed five Pakistanis but failed to penetrate the compound.

In February that year, three American military personnel were among eight killed in a bomb attack at the inauguration of a renovated girls' school in the northwestern district of Lower Dir.

Islamist militants have vowed to avenge American drone strikes targeting them in the semi-autonomous tribal areas and thwart a rumoured prospective Pakistani offensive in North Waziristan.

Although Islamabad is an ally of Washington, relations dramatically worsened last year after a CIA contractor killed two Pakistanis in the eastern city of Lahore and US special forces found and killed bin Laden.

The United States is currently weighing up whether to blacklist as terrorists the Al-Qaeda-linked Haqqani network, whose leaders are understood to be based in Pakistan, in a move that could further damage ties with Islamabad.

Anti-American sentiment in Pakistan has increased since Islamabad agreed in July to end a seven-month blockade on NATO goods crossing into Afghanistan. The blockade was imposed after botched US air strikes killed 24 Pakistani soldiers last November.

Also on Monday, gunmen in southwestern Pakistan attacked three NATO supply trucks, wounding three drivers and torching two of the lorries, police said.