There are signs that Nigeria's Boko Haram jihadis are sending fighters to join the Islamic State in Libya and of increased cooperation between the two groups, a senior U.S. official said on Friday.

Nigeria has asked the United States to sell it aircraft to fight Boko Haram, which has been waging a seven-year insurgency in the north and last year pledged loyalty to the Islamic State, which is active in Syria, Iraq and Libya.

Little is known about the extent of cooperation between the two radical Islamist groups. But Western governments worry that the Islamic State's growing presence in North Africa and ties with Boko Haram could herald a push south into the vast, lawless Sahel region and create a springboard for wider attacks.