U.S. surveillance aircraft are searching for the Nigerian schoolgirls that were abducted by the terrorist group Boko Haram, according to Reuters.

In addition to the U.S. planes, thousands of Nigerian troops are said to have been joined by U.S. and British troops on the ground to search for the girls.

The girls are believed to be in the Boko Haram stronghold in the Sambisa Forest, a game reserve in northeast Nigeria. The forest is about 23,000 square miles -- roughly the size of West Virginia -- with a harsh terrain and many places to hide.

The improved search comes one day after Boko Haram's leader, Abubakar Shekau, released a video offering to release the girls in exchange for its terrorist members that were currently in prison.

A mother of one of the abducted girls recognized her daughter in the video, giving hope that the girls were still alive, although it was unclear when the video had been filmed. The video showed the girls sitting down and wearing veils, but they were still recognizable.

Shekau previously had released a video threatening to sell the girls as slaves.

Nearly 300 schoolgirls have been kidnapped from Nigeria’s northeastern state of Borno since mid-April, prompting international action. Some have escaped, but about 200 remain in captivity.