Clashes between Afghan and Pakistani border guards that left more than a dozen dead will further strain ties between the two neighbours, Kabul said on Tuesday. The fighting erupted on Sunday and continued into Monday, when a NATO soldier and a Pakistani trooper were killed in an ambush after a meeting on the Pakistani side of the border aimed at calming the tension. Pakistan has said the gunman was “unidentified”, but Afghan President Hamid Karzai’s senior spokesman Karim Rahimi repeated on Monday the claim that “a Pakistani officer” had opened fire on the group.Islamabad has denied the allegation. “Such incidents will have unpleasant effects on the relations between the two countries,” Rahimi told reporters in Kabul, also accusing Pakistani guards of initiating the fighting at the weekend. The Pakistani military has said the other side triggered the clashes which involved heavy weapons and left 13 Afghans dead, about half of them policemen and the rest civilians, including children. The Afghan government has complained to the United Nations in a letter “strongly opposing this flagrant interference and irresponsible action by the Pakistani army,” the Foreign Ministry said. On Monday it summoned Pakistan’s ambassador accusing the neighbouring army of an “intrusion” into Afghan territory and of being provocative. “Such incidents will no doubt affect our relations and are against the international norms and against agreements between the two brotherly countries to fight together against terrorism,” the spokesman said. “This is a very serious issue. We are trying through different diplomatic channels to assess the situation (and) to see the reason behind that,” Rahimi said. The spokesman said his government was making efforts to see that similar incidents are avoided, but added that his country would be resolute. “Afghanistan is ready to defend every inch of its soil. Our police are weak but they bravely defended their positions,” he said.