Pakistan hit by claim that army agreed to Nato strike

Pakistan's army was on the defensive yesterday, trying to explain why its air force had not reacted to the killing of 24 troops by Nato as it was confronted with a version of events from US officials that starkly differed from the one it has presented.

The Nato air strikes on 26 November have provoked a crisis between Washington and Islamabad. The Pakistanis have already decided to block Nato's supply routes through the country, to order the CIA to evacuate an airbase and to boycott this week's Bonn conference on regional security.

In a further development yesterday, it was reported that Nato is planning a substantial offensive against militants in eastern Afghanistan. The increase in aggression is likely to stoke tensions between Nato and Pakistan, with cross-border raids into the country and an increase in the use of drones and other air power expected as part of the campaign.

Nato is believed to be targeting several Pakistan-based groups, which have gradually increased their activities in Afghanistan, in an effort to contain them before the handover to Afghan security forces in 2014. Nato and the US have expressed their condolences for the lives lost in last Saturday's attack, but have resisted offering an apology as they wait for the outcome of an investigation.

Some details of the US's own account have leaked out. The Wall Street Journal reported yesterday that a Pakistani liaison officer at one of the border coordination centres the two sides use to track their efforts jointly gave Nato the all-clear to mount the air strike.

The Pakistani officer, the newspaper reported, was apparently unaware that his own troops were located at the target. If true, the claim alters the Pakistani narrative which alleges that it was an "unprovoked act of blatant aggression".

In a statement, the Pakistan army pushed back against the report, saying that it "categorically" denies this version of events. "Wrong information about the area of operation was provided to Pakistan officials a few minutes before the strike," the statement said. "Without getting clearance from Pakistan side, the post had already been engaged by US helicopters and fighter jets," it continued. The involvement of fighter jets is disputed. At a briefing, the Pakistan army's director general of military operations, Major-General Nadeem Ishfaq, said that there were up to three Nato helicopters involved in the air strikes.

It is unclear where the initial burst of fire – which Nato says was from the Pakistani side of the border – came from. US officials told The Wall Street Journal that Afghan forces and American commandos were pursuing Taliban militants when they came under fire from an "encampment along the Afghan-Pakistan border".

The Pakistanis say that they were told of the incident before the air strikes, but deny that any fire emanated from their side. According to their account, the incident only came to an end after Major-General Ishfaq told a Nato commander to pull back the helicopters. It is, however, a mystery why the Pakistanis did not call in their own air force during what was claimed to be a two-hour assault. Similar questions were asked after the 2 May raid that killed Osama bin Laden, when Pakistan said it "scrambled the jets" but was not able to reach what they considered to be helicopters violating their sovereignty.

Meanwhile, Scotland Yard said yesterday that 22 people were arrested outside the US embassy in London during a protest over use of drones in Pakistan organised by a group calling itself United Ummah.

Police said the people were arrested on suspicion of being members of a group banned by the Home Office. It would not say what group it believed the suspects belong to. One person was arrested for violent disorder and another for obstruction. United Ummah does not appear on the Home Office's list of banned groups, but proscribed organisations have rebranded themselves after they were featured.