Sixteen people have been killed in north-west Pakistan in one of the most lethal CIA drone strikes for many months, according to a government official.

The strike came shortly after midnight on Tuesday in North Waziristan, one of the seven semi-autonomous tribal "agencies" near the Afghan border where many of the world's most dangerous militants operate from.

The Pakistani government "strongly condemned" the attack, saying it was a "violation of Pakistan's sovereignty and territorial integrity".

One official from the Federally Administered Tribal Areas (FATA) secretariat said the strike had been on a building used as an unofficial sharia court for dispensing Islamic justice on the outskirts of Miranshah, a town that is also the political capital of North Waziristan.

Four missiles were said to have been fired at the building by one of the unmanned aircraft that operate in the lawless region.

The dead are thought to be a mix of local militants, others from elsewhere in Pakistan and foreign jihadis.

Accurately reporting drone strikes, particularly important details such as the number of people killed and their identities, is notoriously difficult. As with most other strikes, militants in Wednesday's strike reportedly cordoned off the area where the missile struck, making it impossible for local journalists or independent observers to visit the site.

Nonetheless, FATA officials claimed that the targets were militants from the Haqqani Network, a powerful Taliban faction that focuses nearly all of its efforts on attacking Nato and government targets inside Afghanistan.

The Pakistani military has avoided direct confrontation with militants in North Waziristan, while the Haqqani Network was once described by a former US military chief as a "veritable arm" of the Pakistani army's intelligence agency.

The strike was only the second since the US president, Barack Obama, talked openly at the end of May about the programme for the first time. He vowed to tighten the rules governing drone strikes but warned that attacks would have to continue in Pakistan has long as US troops remained in Afghanistan.

The tempo of strikes has fallen dramatically in the past 12 months and it was just the second attack during the premiership of Nawaz Sharif, the Pakistani politician who became prime minister on 5 June.

His party is officially committed to ending the US drone war and formally complained after a strike on 7 June killed a reported seven militants.

In response to that attack, a senior US diplomat was summoned to the foreign ministry in Islamabad and Sharif used his inaugural speech in parliament to demand the end of the drone campaign.

Critics of Pakistan believe that the country continues to give its consent to the drone programme, as it has done the past when it allowed US drones to be stationed at airbases in its territory.

In a BBC interview on Wednesday, the Afghan army chief Sher Mohammad Karimi claimed that Pakistan was complicit in the strikes despite its public denunciations.

Also on Wednesday, six policemen were killed when militants launched a pre-dawn attack on a police post 15 miles from the city of Peshawar. Ten policemen were wounded in the attack during a gun battle that lasted several hours.