US surveillance aircraft were flying over remote areas of northeast Nigeria as part of an international hunt for about 200 girls kidnapped a month ago by Boko Haram militants who stormed their school.
Thousands of Nigerian troops have been sent to the region, while the United States and Britain also have teams on the ground to help with the search.
The mass abduction of the girls from their boarding school in Chibok has caused international outrage and Nigerian President Goodluck Jonathan's government has faced criticism from parents and others, for its a slow response.
It has also brought global publicity to Boko Haram, which has killed killed thousands of Nigerians since it took up arms in 2009 to fight for an Islamist state.
The group had initially threatened to sell the girls into slavery but on Monday offered to trade them for detained militants.
The US State Department said Washington had sent military, law-enforcement and development experts to Nigeria to help search for the missing girls.
"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," a US official said.
US State Department spokeswoman Jen Psaki said US teams on the ground "are digging in on the search and coordinating closely with the Nigerian government as well as international partners and allies".
The girls' exact whereabouts and whether they are being held in one or more groups is not known. Chibok is close to Nigeria's border with Cameroon, Niger and Chad in a sparsely populated area of the Sahel region.
NO SOLDIERS SEEN
Boko Haram leader Abubakar Shekau posted a video on Monday offering to release the girls in exchange for prisoners held by the government - the first visual evidence of them in captivity.
The government said in response that it was exploring all options.
The video showed more than 110 girls sitting on the ground in a rural location chanting, singing and wearing veils. It was not clear when it was filmed or whether Shekau, who sat in front of a green backdrop holding an AK-47 during part of the video, was in the same location as the girls.
(READ MORE: Nigeria's Boko Haram offers to swap kidnapped girls for prisoners)
A mother of one of the girls said had watched the video on television on Monday evening and spotted her daughter among the girls sitting on the ground, said Dumoma Mpur, parent-teachers association chairman at Government Girls Secondary School in Chibok.
"The video got parents apprehensive again after watching it but the various steps taken by the governments and the coming of the foreign troops is boosting our spirit," Mpur told Reuters.
Mpur said he was yet to see a soldier involved in the hunt in Chibok, though he said what looked like a surveillance jet and two helicopters were visible over the area.
A Nigerian soldier with an artillery unit said rebels at the weekend ambushed his unit in the Sambisa forest, which is a Boko Haram stronghold, using mines and rocket-propelled grenades.
(READ MORE: Boko Haram exploits Nigeria's slow military decline)
They killed eight soldiers and wounded four more, said the soldier, who spoke by telephone on condition of anonymity. There was no immediate response from Nigerian Defence Headquarters.
Jonathan was due to return to Abuja on Tuesday from the Congo Republic, where he held talks with President Denis Sassou before a regional security summit in Paris on Saturday to discuss Boko Haram.