A US drone fired four missiles into a vehicle into an Al-Qaeda and Taliban stronghold of Pakistan's northwest tribal belt Monday, killing two militants, local security officials said.

The attack came just hours after Pakistani Taliban chief Hakimullah Mehsud vowed to attack US cities in two purported new videos released months after his reported killing in a US missile strike last January.

His faction also claimed responsibility for an attempted car bombing in New York City and the emergence of the videos spotlight the militant threat in nuclear-armed Pakistan, on the front line of the US-led war on Al-Qaeda.

The missiles destroyed the vehicle in the Marcikhel area, a Taliban and Al-Qaeda stronghold about 20 kilometres (12 miles) east of Miranshah, the main town in the mountainous district.

"US drone fired four missiles targeting a vehicle in the area. Those killed in the attack were militants. We are still checking the death toll and possible injured people," security official told AFP.

"An investigation is on to identify the militants killed in the attack," the first official added.

It was the second strike in a week, as the United States pursues Taliban fighters and other Al-Qaeda-linked groups that have carved out strongholds in the rugged North Waziristan district on the Afghan border.

Another security official confirmed Monday's strike and death toll, speaking on condition of anonymity because he was not authorised to speak publicly.

Local security officials in North Waziristan said the vehicle was parked outside a house at the time of the attack around 3:30 pm (1030 GMT).

The United States has significantly increased drone strikes on North Waziristan this year, the last one last Monday killing eight militants.

More than 890 people have been killed in nearly 100 drone strikes in Pakistan since August 2008, and the bombing raids fuel anti-American sentiment in Muslim Pakistan and draw public condemnation from the government.

But officials in Washington say the strikes are a vital tool to protect the approximately 126,000 foreign troops in Afghanistan, and have killed a number of high-value targets including Pakistani Taliban chief Baitullah Mehsud.

The rugged tribal belt has become a stronghold for hundreds of extremists who fled neighbouring Afghanistan after the US-led invasion in late 2001.

Under US pressure, Pakistan has in the past year significantly increased operations against militants in the area, which Washington has called the most dangerous region on Earth and a global headquarters of Al-Qaeda.

The military has, however, appeared reluctant to launch a similar all-out assault into North Waziristan, where many insurgents have sought sanctuary.