United States surveillance aircraft have joined in the search to find over 200 Nigerian schoolgirls who were kidnapped by extremist group Boko Haram.

According to CBS News, American spy planes are combing through parts of Nigeria in hopes of locating the girls who were abducted from the Government Secondary School on April 14.

Reuters reported that thousands of Nigerian troops have been sent to the region where the girls were taken. In addition, the U.S and the UK have teams on the ground to assist with the search.

U.S. State Department spokeswoman Jen Psaki said the teams on the ground “are digging in on the search and coordinating closely with the Nigerian government as well as international partners and allies.”

On Monday, a video from Boko Haram was released in which suspected leader, Abubakar Shekau, said that he would not release the girls unless all militant prisoners are freed.

Shekau also said that the schoolgirls have converted to Islam and the video showed many of them in traditional Muslim garb praying and chanting.

The exact whereabouts of the girls are still unknown at this time.