A Taliban suicide bomber and gunmen attacked a restaurant popular with foreigners in the Afghan capital, killing up to 21 people including three United Nations staff and the IMF's top representative in Afghanistan.Gunmen burst into the restaurant spraying diners with bullets after the bomber blew himself up near the entrance around 7.30 p.m. on Friday evening.Thirteen foreigners were among those killed, according to police, and details of the victims began to trickle through on Saturday.The U.S. embassy said in a post on Twitter that at least two U.S. private citizens were killed. Britain and Canada confirmed they had each lost two nationals, and Denmark said one of its citizens also died.After the initial blast, sporadic bursts of gunfire were heard over the next hour. The two gunmen inside the Lebanese restaurant, located in Kabul's diplomatic enclave, were shot dead by police, an Afghan official said.Most foreign forces are preparing to leave Afghanistan this year after more than a decade of war, and there are fears that the Taliban will intensify attacks in the run up to an election in April to find a successor to President Hamid Karzai.At odds with Washington over the terms, Karzai is still deliberating whether to allow some U.S. troops to stay on. If no agreement is reached, Afghan forces could be left to fight the insurgents on their own.The Taliban claimed responsibility for Friday's attack, calling it revenge for a U.S. airstrike earlier this week that had also drawn condemnation from Karzai as eight civilians were killed.