The Nigerian government has signalled it is ready to negotiate with the Islamist militants who are holding more than 200 schoolgirls captive, as international assistance to the search and rescue efforts intensifies.
Canada is the latest country to disclose that it has sent special forces to Nigeria, joining teams from the US, UK, France and Israel.
The Canadian prime minister, Stephen Harper, said his country's forces would not engage in combat but were "to provide liaison and to assist Nigerian authorities in their search".
Britain has offered surveillance aircraft and a military team, David Cameron told parliament on Wednesday. The prime minister said: "Today I can announce we have offered Nigeria further assistance in terms of surveillance aircraft, a military team to embed with the Nigerian army in their HQ, and a team to work with US experts to analyse information on the girls' location."
The abduction of the girls was, Cameron said, "an act of pure evil", adding: "The world is coming together not just to condemn it but to do everything we can to help the Nigerians find these young girls."
British Foreign Office minister Mark Simmonds will meet officials in Abuja on Wednesday to discuss further assistance. A UK team of military advisers and family liaison officers, led by Brigadier Ivan Jones, has been in Nigeria since Friday.
In Abuja, the special duties minister, Taminu Turaki, said the government was open to talks with Boko Haram, the Islamist group that abducted the girls a month ago from their school in Chibok, in the north-east of the country.
"Nigeria has always been willing to dialogue with the insurgents," he told AFP. "We are willing to carry that dialogue on any issue, including girls kidnapped in Chibok."
A group of about 130 of the kidnapped girls appeared on a video released this week by Boko Haram, After a special viewing for parents, all the girls were confirmed as students of the Government Girls secondary school in Chibok. Although most of the abducted girls are Christian, all were wearing Muslim dress and two were singled out to say they had converted to Islam.
The Boko Haram leader, Abubakar Shekau, said the girls could be released in exchange for jailed militants. "I swear to almighty Allah, you will not see them again until you release our brothers that you have captured," he said in the video.
Amid fears that the girls have been forced into sex slavery or will be trafficked to other countries, international assistance to the Nigerian government is mounting.
The US has launched airborne surveillance missions over the Sambisa forest, where the girls are believed to be being held. It is likely that the US and UK are offering the Nigerians sophisticated surveillance and eavesdropping technology as well as satellite imagery.
France is to host a summit on Saturday with representatives from several African countries, including Nigeria, to discuss ways of dealing with the threat from Boko Haram.
Israeli experts are thought to be in Nigeria following last weekend's offer of help by the prime minister, Binyamin Netanyahu, to President Goodluck Jonathan. A spokesman for Netanyahu declined to give further details.
China has also offered assistance. Colonel Ku Hang Li, the Chinese defence attache to Nigeria, said the government would do everything in its power but declined to give details of its security intervention.
