Security has been tightened across the Afghan capital, Kabul, following Sunday\'s bomb explosion outside the offices of an American security firm. Nato-led peacekeepers say at least three Americans and three Afghans died in the blast, claimed by the Taleban. But there is confusion about the final casualty toll, with reports of several more dead including some Nepalis. Kabul is tense with extra checkpoints and troops patrolling the streets, and increased security outside embassies. The US embassy and other international organisations in Kabul have told staff to avoid all but essential movement in the city following the blast. The BBC\'s Andrew North in Kabul says that some embassies now have international peacekeeping troops outside in addition to their usual guards. Security was especially tight around another building in the city, linked to Dyncorp, the US firm hit by Sunday\'s explosion. Afghan President Hamid Karzai says he is deeply disturbed by the attack. Dyncorp provides his bodyguards and trains the new Afghan police force. (BBC)