ON Friday, the Nato commander in Afghanistan met the Pakistani army chief in Islamabad to discuss communication and coordination across the Pak-Afghan border. Less than 24 hours later, troops at a Pakistani border check post in Mohmand became targets of Nato fire. Twenty-four of them died, according to the Pakistan military, and the incident appears to be the deadliest of its kind; a similar strike in September 2010 took the lives of two soldiers and another in June 2008 killed 11.

The Pakistani administration's reaction seemed designed not just to handle the issue domestically but also to send a strong — and justified — signal to the US that this matter was not taken lightly. There was a flurry of activity in Islamabad and Rawalpindi on Saturday, with the prime minister placing phone calls to major political parties and rushing back from Multan to convene an emergency meeting of the cabinet's defence committee. Gen Kayani called a meeting of senior army officers and declared the incident unacceptable, disapproval was conveyed in Washington and Brussels, and Nato supply routes to Afghanistan were blocked. Given the nature of the incident, the reaction was entirely warranted. Previous such incidents have been described as the result of miscommunication or of mistakes that took place during the pursuit, or perceived pursuit, of militants. And it is true that Mohmand is an area through which militants do cross the border. But the fact that the strike was aimed at a military check post, and that a large group was targeted (there were reportedly about 40 soldiers at the post), makes that a tenuous explanation in this case. Only a thorough probe can settle the issue, and in promising one the Isaf chief has done the right thing. But Pakistan must be included in the investigation, and if a mistake was made, Nato must commit to ensuring that its troops will not repeat it in the future. Regardless of the outcome, a formal apology is also required.

Coming at a time when US-Pakistan ties had barely begun to recover, if at all, the incident's timing is also particularly unfortunate for that relationship. Secretary of State Hillary Clinton's visit last month had begun to improve the atmosphere somewhat after the latest controversy over the Haqqani network, but this threatens to sour relations and derail cooperation. The Pak-Afghan border has become increasingly dangerous for both countries, with incursions and attacks taking place in both directions. The need is for more cooperation, not less. But the mistrust that an incident like this can foster will do nothing to bring that about.