US Secretary of State John Kerry says a specialist team is set to start work in Nigeria to help find more than 200 schoolgirls abducted by militants.
He said the US team, working with the Nigerian government, would do everything possible to free the girls.
Earlier, Nigerian President Goodluck Jonathan said he hoped “a turning point” had been reached in the fight against Islamist insurgents Boko Haram.
Nigeria has been criticised for its slow response to the kidnappings.
The schoolgirls were seized from their boarding school on the night of 14 April in the town of Chibok in north-eastern Borno state.
It is believed they are being held somewhere in the vast forested areas that stretch from near Chibok into neighbouring Cameroon.
“Our inter-agency team is hitting the ground in Nigeria now and they are going to be working in concert with President Goodluck Jonathan’s government to do everything that we possibly can to return these girls to their families and their communities,” Kerry said.
“We are also going to do everything possible to counter the menace of Boko Haram.”
On Wednesday, President Barack Obama said the team comprised personnel from military, law enforcement and other agencies.
He said he hoped the kidnapping would galvanise the international community to take action against Boko Haram.
President Jonathan, speaking at the World Economic Forum being hosted in the capital, Abuja, said the abduction of the girls could be a turning point in the battle against Boko Haram.
“I believe that the kidnap of these girls will be the beginning of the end of terror in Nigeria,” he said.
In his speech, he also thanked China, the US, the UK and France for their offers of help to rescue the girls.
France on Thursday announced it would station about 3,000 troops in Nigeria’s neighbours to help tackle Islamist militants in the Sahel region.
Shortly after his speech, President Jonathan told the BBC that with the assistance his country was now receiving, “we will be able to bring terror to an end in Nigeria”.
Boko Haram, whose name means “Western education is forbidden” in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence and security crackdown this year alone.
In a video released earlier this week, Boko Haram leader Abubakar Shekau threatened to “sell” the students.
He said they should not have been in school in the first place, but rather should get married.
The abductions have triggered a growing social media campaign with the hashtag #BringBackOurGirls backed by public figures and celebrities.
US First Lady Michelle Obama has joined in, tweeting: “Our prayers are with the missing Nigerian girls and their families. It’s time to #BringBackOurGirls.”
On Thursday, US actress Angelina Jolie blamed a “culture of impunity” for the kidnapping. She said the world had to “make sure this stops happening”.
Malala Yousafzai, the Pakistani schoolgirl who survived a shooting by Taliban insurgents, has also spoken out, saying the world must not stay silent over the abduction.
She told the BBC that “if we remain silent then this will spread, this will happen more and more and more”.
