Abuja - Britain advised its citizens on Wednesday against travelling to several regions in northern Nigeria, after an increase in attacks blamed on Islamist militants and the abduction of several foreigners earlier this month.
Gunmen killed a security guard and abducted a Briton, an Italian, a Greek and four Lebanese workers after storming the compound of Lebanese construction firm Setraco in Bauchi state on February 16.
It was the worst case of foreigners being kidnapped in the mostly Muslim north of Africa's most populous country since an insurgency by Islamist militants intensified two years ago.
Britain upped its travel risk ratings on Wednesday, advising against any travel to Bauchi state and Okene city in southern Kogi state where militants last month attacked Nigerian troops who were bound for Mali to counter an Islamist insurgency.
It also advised against "against all-but-essential travel" to Kaduna, Kano, Jigawa and Katsina states, a statement from the foreign office said.
Attacks by Islamist groups in northern Nigeria have become the biggest threat to stability in Africa's top oil producer.
Western governments are concerned the militants may link up with groups elsewhere in the region, including al Qaeda's North African wing AQIM, especially given the conflict in nearby Mali. France sent troops to Mali last month to help oust Islamist rebels.
Islamist group Ansaru claimed responsibility for the Setraco raid in Bauchi and the Okene attack. The Setraco raid was "based on the transgression and atrocities done to the religion of Allah by the European countries in many places such as Afghanistan and Mali," said the group, which has kidnapped other foreigners in Nigeria in the past.
Britain put Ansaru on its official "terrorist group" list in November , saying it was aligned with al Qaeda and was behind the abduction of a Briton and an Italian killed last year during a failed rescue attempt.
The group's full name is Jama'atu Ansarul Musilimina Fi Biladis Sudan, which roughly translates as "Vanguards for the Protection of Muslims in Black Africa".
It is believed to be a breakaway group from better known Islamist sect Boko Haram, which has killed hundreds in recent months in its attempt to carve out an Islamic state in a country split roughly equally between Christians and Muslims.
Violence is stunting economic development in the north and risks increasing the divide with the wealthier and largely Christian south, which is home to the commercial hub Lagos and oil-producing Niger Delta region.
The Nigerian military in Boko Haram's base in far northeast Borno state said on Wednesday that soldiers had killed a commander of the sect and three other members this week.