He made the comments as he held a sign bearing the "#Bring Back Our Girls" slogan on the BBC's Andrew Marr show.

Mr Cameron is the latest high-profile supporter of the social media campaign after US First Lady Michelle Obama was pictured with a similar poster.

The Islamist militant group Boko Haram has claimed the abductions.

During the programme, fellow guest Christiane Amanpour, CNN's chief international correspondent, handed Mr Cameron the sign and asked if he would like to join the campaign.

Taking it from her, he replied: "Happily."

Mr Cameron later tweeted: "Proud to support #BringBackOurGirls."

He told the BBC One programme: "I rang the Nigerian president to offer anything that would be helpful and we agreed to send out a team that includes some counter-terrorism and intelligence experts to work alongside the bigger American team that's going out there.

"We stand ready to do anything more that the Nigerians would want."

He said it was unlikely Nigeria would ask for British troops to help but added: "I said to President Jonathan where we can help, please ask, and we will see what we can do."

Michelle Obama has been campaigning for the release of the girls

Mr Cameron also spoke of the importance of tackling extremism around the world.

"This is not just a problem in Nigeria," he said. "We're seeing this really violent extreme Islamism - we see problems in Pakistan, we see problems in other parts of Africa, problems in the Middle East.

"Also, let's be frank, here in the UK there is still too much support for extremism that we have to tackle, whether it's in schools or colleges or universities or wherever."

He recognised it was not an easy task to look for the girls, who were taken from their school in Chibok on 14 April.

Continue reading the main story

Boko Haram at a glance

• Founded in 2002
• Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the local Hausa language
• Launched military operations in 2009 to create Islamic state
• Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja
• Some three million people affected
• Declared terrorist group by US in 2013

• Profile: Boko Haram leader Abubakar Shekau
• Who are Boko Haram?
• Abduction timeline

"We can't just pile in and do whatever we'd like," said Mr Cameron. "It's immensely complicated because they are probably in this deep area of jungle that is three times the size of Wales.

"But it's good that efforts are being stepped up and we'll do what we can."

The Foreign Office has said there are "large information gaps" because of the scale and nature of the incident.

"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," a spokesman said.

Mrs Obama has described herself and Barack Obama as being "outraged and heartbroken" over the girls' abduction.

Speaking instead of her husband in the weekly presidential address, she said: "What happened in Nigeria was not an isolated incident. It's a story we see every day as girls around the world risk their lives to pursue their ambitions."

The Pope has also tweeted his support, writing: "Let us all join in prayer for the immediate release of the schoolgirls kidnapped in Nigeria. #BringBackOurGirls."

During Sunday's programme, Amanpour told Marr about the social media drive, saying: "On the one hand, the hashtag is great because it mobilises people.

"On the other hand, I'm a tiny bit concerned it's a big bubble that then collapses with nothing really being done."

However she said Nigeria is "100% saturated with social media" and that "this (campaign) is really getting to the people in Nigeria".