U.S. still on high alert for Nigeria attacks ABUJA (Reuters) - The United States remains on high alert attacks on its interests in Nigeria by Islamists, even though they have not mounted any major operations for months, the U.S. ambassador said on Thursday. Ambassador Terence McCulley also told Reuters that unrest in northern Mali made it easier for Nigeria's Islamist sect Boko Haram to link up with outside jihadist groups, including al Qaeda's north African wing. We're still on a pretty high state of alert. There are potential targets of opportunity so ... we need to be careful," McCulley said in an interview in the Nigerian capital Abuja. Boko Haram is fighting to impose Islamic, or sharia, law on Nigeria, whose population of 160 million people is split evenly between Christians and Muslims. At least 2,800 people have died in fighting in the largely Muslim north since the sect launched an uprising against the government in 2009, watchdog group Human Rights Watch says. McCulley alluded to a bombing claimed by Boko Haram of the Nigeria headquarters of the United Nations in August 2011, killing 24 people, and the kidnappings and killings of one British, one Italian and one German citizen earlier this year. Although Boko Haram itself denied any involvement in the kidnappings, they were thought to be the work of other Islamist groups with links to the sect. That is clear evidence that if there are targets of opportunity they will seize them. They are working contrary to our interests and other Western interests," McCulley said. Threats to U.S. interests by Nigerian radicals were highlighted on Christmas 2009, when Nigerian Umar Farouk Abdulmutallab tried to blow up a U.S. airliner bound for Detroit with a bomb hidden in his underpants. McCulley said Nigerian forces had made some headway against the insurgency, including taking out some leaders, and for months there had not been an attack with massive casualties, like the one in Kano that killed 186 people in January. "I don't think that diminishes the group's intent to carry out these types of acts so I think we have to be careful in assuming the insurgency has ... been tamed," McCulley said. Apart from the U.N. bombing and the kidnappings, Nigerian Islamists have seemed more interested in local targets. McCulley said chaos in Mali had enabled Boko Haram to forge links with jihadists who have a global, anti-Western agenda. Military experts from Africa, the United Nations and Europe have drafted plans to retake control of northern Mali, but no action is expected until well into next year. Security officials say there is evidence some Boko Haram members have received training from al Qaeda in the Islamic Maghreb (AQIM). The U.N. bombing last year raised concerns that the sect was using its training to target Western interests. "An Islamist enclave in a vast northern Mali certainly makes that easier ... given porosity of borders and the proliferation of arms in the region," McCulley said. It gives one cause for concern that this could be a significant problem in the future. Editing by Tim Cocks and Michael Roddy