Washington, Oct 15 (IANS): A US aid worker was kidnapped by armed assailants from outside his home after they killed his bodyguard and a police officer in Niger, media reports said.

The abductors forced the aid worker in Abalak to strip down to his undergarments before putting him in their vehicle and fleeing north toward neighbouring Mali, CNN quoted a source as saying.

According to the source, this was usual practice by terrorist groups trying to avoid hostages being tracked.

Authorities have been taking all necessary measures to locate the US citizen, including imposing a heavy military presence between Abalak and the border with Mali, the source said.

It was the first time a foreigner had been kidnapped in the area, the government source said.

No group has claimed responsibility so far.

The US State Department said it was aware of reports of the kidnapping.

"The safety and security of US citizens overseas is among our highest priorities. We have no further information to offer at this time," a spokesperson said.

The State Department has warned against travel to Niger due to the threat from terrorist groups in the region.

"Terrorist groups have called for and executed attacks against countries that supported the intervention against terrorist groups in northern Mali, including Niger," its advisory said.

Boko Haram is active in the country and there is also a threat from Al Qaeda, it said.

"The terrorist organisation Al Qaeda in the Islamic Maghreb (AQIM) has kidnapped Europeans in the region and continues to threaten to kidnap Westerners, including US citizens, in Niger," the State Department warned.