Prime Minister David Cameron has not ruled out the possibility that British troops could be used to help rescue the kidnapped Nigerian schoolgirls.

When asked on BBC One's Andrew Marr Show, he said: "We stand ready to do anything more that the Nigerians would want.

"We can't just pile in and do whatever we would like. It is immensely complicated because they are probably in this deep area of jungle that is three times the size of Wales, but it is good that efforts are being stepped up and we will do what we can."

Nigerian president Goodluck Jonathan and his government has been hit by stinging criticism, both at home and abroad, for being too slow to react after the 276 girls were snatched from their school in Chibok by Islamic extremists Boko Haram on April 14.

Asked whether British troops might be used to help rescue the girls if Nigeria asked, Mr Cameron told the programme: "I think they are unlikely to ask for British troops but we have worked with the Nigerians in the past, in hostage rescue situations where British special forces have helped and advised and the rest of it, and so I said to President Jonathan, where we can help please ask and we'll see what we can do.

"He accepted the idea of a team (of UK experts) to go out and advise and help, even before that British helpers and advisers have been working with Nigerian police and military on other issues so they know us, they have worked with us."

A UK team, now among a range of international experts on the ground to help find the girls, have admitted they face "large information gaps".

The British team, who are in the Nigerian capital Abuja, include counter-terrorism and intelligence experts and are working alongside their American counterparts.

Mr Cameron also publicly backed the #BringBackOurGirls campaign by holding up a sign with the message on the show.

The Twitter campaign has gone viral and helped alert the world to the agony of the Chibok families as well as to build outrage towards Boko Haram, which has threatened to sell the girls into slavery and uses kidnapping and violence as its signature weapons.

US First Lady Michelle Obama yesterday said she was outraged and heartbroken by the abductions, as she took the rare step of making outspoken foreign policy remarks about the kidnappings.

In delivering husband Barack Obama's weekly video address, she condemned the "unconscionable" act, saying it was carried out by "a terrorist group determined to keep these girls from getting an education".

"Like millions of people across the globe, my husband and I are outraged and heartbroken," she said.

Demonstrations in support of the missing Nigerian girls have been held around the world, while the social media campaign continues to grow.

Mrs Obama and girls' education campaigner Malala Yousafzai are among hundreds of people who have tweeted a photo of themselves with a sign reading #BringBackOurGirls in a show of support.

According to reports, the search is closing in on a forest near the border with Cameroon and the girls have been divided into at least four groups - which would make a rescue raid more difficult.

Boko Haram has staged many attacks in north-eastern Nigeria over the years, a campaign of bombings and massacres that has intensified in recent times despite a strong military offensive.

Since May last year, there has been a state of emergency in three north-eastern Nigerian states covering one-sixth of the country.

Boko Haram has killed more than 1,500 people this year, compared to an estimated 3,600 between 2010 and 2013.

The Socio-Economic Rights and Accountability Project, a leading Nigerian rights group, has urged the UN Security Council to impose sanctions against Boko Haram , saying concern and condemnation are not enough.

It is time for the council to "act decisively", they said, and the cost of inaction is "too high to contemplate".

Meanwhile, more international experts are due to arrive in Nigeria to help in the search, including US hostage negotiators.

The British team, which includes officials from the Department for International Development, the Foreign and Commonwealth Office (FCO), the Ministry of Defence (MoD) and the Metropolitan Police, are working with specialists from the US and other countries to co-ordinate search efforts. China, France and Spain have also promised help.

An FCO spokesman said: "The scale and complexity of the incident and the environment means there are large information gaps.

"The priority for the team in the first instance is establishing the facts, such as the precise identities of those taken and what has actually happened, to help Nigeria build a better picture."

The MoD has already linked up with the Nigerian armed forces, including the military which has operational control of the region where the kidnaps took place. There has been a meeting with the Americans to determine joint arrangements, areas of responsibility and what the next moves may be.

Metropolitan Police officers, which include the family liaison officers personally requested by Mr Jonathan, have met with Nigerian police to talk about how to tackle the huge demand for victim support.

Members of the UK team have also seen first-hand the pain and distress caused by the mass kidnapping in a meeting with a group representing the Chibok families, the FCO spokesman said.

Mounting international outrage at the mass kidnapping has increased after an alarming report by Amnesty International claimed Nigerian commanders were warned armed men were beginning to arrive near Chibok, but the military were unable to raise enough troops to respond.

Nigerian security forces had four hours of notice about the attack but did not react because of their fear of engaging the extremists, according to the Amnesty International report.

The PM's support for the #BringBackOurGirls campaign immediately drew criticism on Twitter.

Mr Cameron said he would "happily" join the campaign when asked to by CNN journalist Christiane Amanpour, another guest on the Andrew Marr programme. He later tweeted a picture of them both holding a sign with the handwritten words "#Bring Back Our Girls".

But in response, author David Cushman tweeted: "The point of a campaign like #BringBackOurGirls is for people like us to get people like you to do something - not just join," while novelist Jojo Moyes said: "The #BringBackOurGirls hashtag is great. But it worries me that David Cameron thinks holding it up on TV is really the best he can do."