Suspected Boko Haram militants have kidnapped eight more girls in northeastern Nigeria.

The latest kidnapping happened on Sunday night in the village of Warabe, in Borno state. The girls taken were between the ages of 12 and 15.

On Monday, Boko Haram's leader threatened to "sell" more than 230 girls seized from their school, also in Borno, on 14 April.

The Islamist insurgency by Boko Haram has left thousands dead since 2009.

US President Barack Obama has vowed that the United States will do "everything we can" to help save the abducted girls.

The Nigerian government earlier said it welcomed an offer by the US to send a team of experts, including soldiers and hostage negotiators, to help investigate the first abductions.

The BBC's Mansur Liman in Abuja says the area around Warabe, the site of the latest abductions, is a stronghold of the Islamist movement.

The gunmen arrived in two trucks and also seized animals and food from the village.

Communications are very poor in the area, which explains why the news took several days to emerge, our correspondent says.

Residents from a nearby town told AFP that they feared Boko Haram would target them next.

"We have no security here. If the gunmen decide to pick our own girls, nobody can stop them," Peter Gambo said.

Warabe is also close to the Sambisa forest, where the first group of schoolgirls is thought to have been taken.

Boko Haram leader Abubakar Shekau released a video on Monday confirming that his group had abducted them.

"God instructed me to sell them, they are his properties and I will carry out his instructions," he said.

Officials from the US State Department said there was evidence that some of the girls had been moved into neighbouring countries such as Cameroon and Chad.

UN envoy and former British prime minister Gordon Brown has also called on the US and UK to mount a surveillance operation and to help rescue the girls if they find them.

There is growing concern, within and outside Nigeria, at the failure to locate the girls.

Ugandan President Yoweri Museveni, whose military sought to overcome similar tactics by the Lord's Resistance Army, condemned the Boko Haram attacks.

"It would be a mistake for the government of Nigeria to negotiate with these people. The most important thing is to defeat them, then negotiations can come after that," he said.

Boko Haram, which means "Western education is forbidden", has attacked numerous educational institutions in northern Nigeria.

It has also attacked targets in the capital Abuja, with the deadliest attack so far coming on the same day the 200 girls were kidnapped in Chibok.

In that attack, at least 70 people died in a bomb blast in the suburb of Nyanya