The Nigerian government has confirmed that a military operation has been launched with international backing to locate and rescue more than 200 girls, who have been held captive for a month.

The government also signalled that it is ready to negotiate with Boko Haram, the Islamist militant group that snatched the girls from a school in Chibok, in the north-east of the country, although it appeared to rule out any prisoner exchange.

"International operations have commenced in collaboration with the Nigerian military … The operations are being carried out in conjunction with Nigerian troops," Mike Omeri, coordinator of the national information centre, told a press conference.

"Surveillance? Yes. Intelligence? Yes. And knowledge and experience sharing will be applied," he added. He declined to give further details.

As international assistance to the search and rescue efforts intensified, Mark Simmonds, Britain's minister for Africa, held talks with Nigerian president Goodluck Jonathan in the capital, Abuja. Jonathan "made it very clear that there will be no negotiation with Boko Haram that involves a swap of abducted schoolgirls for prisoners," Simmonds said. But Nigeria's government will talk to the militants to seek reconciliation, Simmonds added.

Meanwhile, "The point that also was made very clear to me is that the president was keen to continue and facilitate ongoing dialogue to find a structure and architecture of delivering lasting solution to the conflict and the cause of conflict in northern Nigeria."

David Cameron told parliament on Wednesday that Britain had offered to step up its assistance by sending surveillance aircraft and a military team. A UK team of military advisers and family liaison officers, led by brigadier Ivan Jones, has been in Abuja since Friday.

The prime minister told MPs: "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."

Canada became 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".

In Abuja about 200 people wearing red #BringBackOurGirls T-shirts gathered for a vigil marking 30 days since the girls' abduction. Among the speakers was pastor Enoch Mark of Chibok. "I lost my two daughters," he told the audience. "Today is 30 days. I have not been sleeping well. There is no night from 14 April when I can sleep more than three hours. I am not eating well. But seeing you this evening, you give me hope and you give me courage."

Amid chants of, "Bring back our girls now," a group of teenage girls read out the names of the missing. One said: "The only reason I was not captured that night is that I was not in Chibok."

Hadiza Bala Usman, coordinator of #BringBackOurGirls, earned cheers and a standing ovation as she declared: "I feel empowered as a Nigerian to see people across religious lines, across ethnic lines, standing together. We will say no to insurgency. No one will divide us along political and ethnic lines. We retain these core values as Nigerians."Earlier the special duties minister, Taminu Turaki, said the government was open to talks with Boko Haram. "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.

Omeri refused to confirm that channels of communication with Boko Haram were open. "We said all options are on the table, and we will adopt international best practice in this regard," he said.

He rejected criticism that the government had not yet found the girls. "When the Americans wanted to get their hostages in Iraq, they knew where they were and it took 200 days. We don't want it to take 200 days. One thing I feel certain of: the mystery will be unravelled."

The government believed all the girls were still alive and in Nigerian territory, rejecting reports that some had been sold into slavery.