Prime Minister David Cameron has not ruled out the possibility British troops could be used to help rescue 200 kidnapped Nigerian schoolgirls.

When asked on BBC 1'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, 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 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 working on the ground to help find the girls, has admitted it faces "large information gaps".

The British team in the capital Abuja includes counter-terrorism and intelligence experts and is working alongside its American counterparts.

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

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.

Boko Haram has killed more than 1500 people this year, compared to an estimated 3600 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", it 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.