The Islamist militant group Boko Haram, which is fighting a violent insurgency in northeast Nigeria, has from 4,000 to 6,000 "hardcore" fighters, U.S. intelligence officials said on Friday.

In an assessment of the group, whose five-year uprising has included massacres and kidnappings, and has spread from Nigeria into neighboring states, the officials said they did not believe it posed a major threat to Nigeria's oilfields in the south.

The officials, who spoke on condition of anonymity, said the militants were believed to be still holding about 300 schoolgirls they kidnapped early last year and had dispersed them to multiple locations.

Around 10,000 people were killed in Boko Haram attacks last year. The Sunni Muslim group poses the biggest security threat in Nigeria, Africa's top oil producer and biggest economy.

Concern over the insurgency appears to be one of the main reasons for what appears to be a surge in political support for opposition leader Muhammadu Buhari in an upcoming Feb. 14 election.

Many Nigerians believe Buhari, as a former military ruler, will be able to bolster the army's hapless efforts to counter the insurgency, and that as a Muslim he may even be able to take some of the wind out of Boko Haram's ideological sails.

The officials said the militants had been engaging in both small-scale and larger attacks in recent weeks and they expected this mixed pattern of operations to continue during the election period.

The U.S. intelligence officials said the Nigerian military forces were stretched thin in fighting the insurgents, as well as by their involvement in international peacekeeping forces.

But military forces in neighboring Cameroon, Chad, and Niger, where Boko Haram had spread its attacks, were taking an increasing active role in combating the group.

One official said: "It remains to be seen how much tactical prowess [Boko Haram] have" in fighting regular military forces.

Sources in the region say Chad and Cameroon are deploying thousands of troops and Niger has reinforced its border against the militant group, but they face an uphill battle against a group which has rebuffed offensives by the Nigerian military.

The officials said over the last year, Boko Haram had established a "safe haven" in territory it controlled, which included 30 or more towns and villages.

The group, which says it wants to establish an Islamic state, has produced videos praising the Islamic State militants who have taken over parts of Iraq and Syria.

But one of the U.S. officials said there was "no known tactical cooperation or leadership contact between the two groups."