Surveillance planes fly over remote forest area where more than 200 teenage girls are suspected of being held by Boko Haram

US surveillance planes have begun flying missions over a remote area of Nigeria as part of a mounting international effort to find and rescue more than 200 teenage girls abducted by Islamist militants almost a month ago.

The US has also sent expert advisers to Nigeria to "dig in on the search" and has provided the government with satellite images.

The plight of the schoolgirls and the desperation of their families has captured world attention, with abhorrence focused on Boko Haram, the violent jihadist group that is holding the girls. But there has also been widespread criticism of the Nigerian authorities, whose efforts to find the captives is perceived to be lax.

As well as US assistance, Britain, France and China have sent teams to Nigeria to help the search, and Israel has offered to join the international effort.

State department spokeswoman Jen Psaki said the US was "providing intelligence, surveillance and reconnaissance support". Military and law-enforcement teams on the ground were "digging in on the search and co-ordinating closely with the Nigerian government as well as international partners and allies", she said.

A senior Obama administration official said: "We have shared commercial satellite imagery with the Nigerians and are flying manned ISR [intelligence, surveillance and reconnaissance] assets over Nigeria with the government's permission."

More than 270 girls were snatched by militants from their boarding school in Chibok, north-eastern Nigeria, on the night of 14 April. Some managed to escape, but most were taken into the remote Sambisa forest. Their families fear they have been forced into sex slavery, or will be trafficked to other countries.

On Monday, Boko Haram released a 27-minute video, showing about 130 girls in Muslim dress and reading from the Qur'an. Most of the seized girls are Christians. Two were singled out to tell the camera they had converted to Islam.

In the video, 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.

Nigeria's president, Goodluck Jonathan, said on Sunday that he believed the girls were still in the country, and that international assistance had raised hopes of finding them.

The former British prime minister Gordon Brown, who called for international military assistance two weeks ago and travelled to Nigeria last week, said on Monday: "There will be worldwide condemnation of a new video showing Boko Haram cruelly and barbarically using 200 kidnapped girls to bargain for the release of prisoners and exploiting innocent young girls for political purposes.

"It is urgent that all religious leaders in every part of the world speak out against their perverted and twisted version of Islam which involves forced conversions and the sale of girls as sex slaves."

Binyamin Netanyahu, the Israeli prime minister, offered his country's help in a telephone call to Jonathan on Sunday. "Israel expresses its deep shock at the crime committed against the girls," he said. "We are willing to help assist in locating the girls and fighting the terror that is afflicting you."

High-profile figures around the world have backed the campaign for the girls' release, rallying behind the hashtag #BringBackOurGirls. Michelle Obama took over her husband's weekly presidential address to express "outrage and heartbreak" over the mass abduction, and to pledge US government support for the rescue effort.