(CNN) -- The United States is now using drones and manned surveillance aircraft over Nigeria in the search for the hundreds of schoolgirls kidnapped by Boko Haram.
But signs of them are yet to emerge, and the task is complicated by the possibility that the girls have been separated into groups.
The United States, which is supporting the Nigerian government's efforts, will continue to deepen its efforts, Defense Secretary Chuck Hagel said Wednesday while traveling to Saudi Arabia.
However, "I have seen no intelligence come back that I am aware of that shows that we've located those girls," he said.
The assets provide some of the best tools to try to find the girls, but the Nigerian government has shown some reluctance to use U.S. help.
For now, the United States is not sharing raw intelligence from its surveillance aircraft with Nigeria's armed forces because the countries have still not established the intelligence-sharing protocols and safeguards needed for an intelligence-sharing agreement, Pentagon spokesman Col. Steve Warren said.
That said, the intelligence gathered through the surveillance flights is being fed to an interdisciplinary team on the ground, and that team is analyzing it and providing advice to the Nigerian government, he said.
Warren added that the manned and unmanned aircraft being used are unarmed.
U.S. Secretary of State John Kerry called the kidnapping of hundreds of girls an "unconscionable crime," vowing to do "everything possible to support the Nigerian government to return these young women to their homes and to hold the perpetrators to justice.
"I will tell you, my friends, I have seen this scourge of terror across the planet, and so have you. They don't offer anything except violence," he said in a statement. "They just tell people, 'You have to behave the way we tell you to,' and they will punish you if you don't."
Pressure to find the girls
Wednesday marked one month since the 276 girls were abducted from Chibok by Boko Haram. A worldwide campaign to "bring back our girls" has spread awareness of the incident, and as the days go by, the pressure to find them increases.
At least four Nigerian soldiers died in an ambush Wednesday as they were returning from patrol duties in Chibok, the Nigerian military said.
U.S. Sen. John McCain, R-Arizona, is among those who support American military intervention to find the girls, if needed.
"You know, it's interesting to me that when a ship is hijacked and taken into custody by these pirates, we have ... no reservations about going in and trying to take that ship back and the crew that's being held," he said. "We have no compunctions about that."
When it comes to the hundreds of girls who were kidnapped, the response has dragged, he said.
A U.S. military operation "could be done in a way that is very efficient, but for us not to do that, in my opinion, would be an abrogation of our responsibilities," McCain said.
Two senior administration officials told CNN that it is premature to talk about a special operations incursion into Nigeria because the girls have not been found yet.
The U.S. military is there to advise and assist, but not to actively participate, the sources said.
If the girls are found, it would be up to the Nigerians to devise a plan and execute it with U.S. assistance, the sources said.
And that raises other complications.
The Nigerian military is capable of carrying out a rescue operation, but there are concerns because it has been heavy-handed in the past and killed many civilians, the sources said.
As it currently stands, U.S. law prohibits the U.S. military from working with Nigerian military units that have been accused of abuses, a senior State Department official said.
"We've been very clear about our concerns about the Nigerian reports of and evidence of abuses by the Nigerian military," the official said.
Even with all of these complications, the U.S. is committed to doing everything it can to find the girls, the official said.
Separately, Nigerian President Goodluck Jonathan is not considering swapping Boko Haram prisoners for the kidnapped girls, according to media reports, which cited Mark Simmonds, Britain's Africa minister, who met with the President.
The director of Nigeria's National Orientation Agency, a government information ministry, had previously said negotiations could be an option.
CNN's Elise Labott, Jim Acosta, Paul Courson, Vlad Duthiers and Shirley Henry contributed to this report.