Updated at 7:00 p.m.
Defense Secretary Chuck Hagel told CBS News State Department Correspondent Margaret Brennan that the U.S. is doing "everything possible" to locate the missing Nigerian girls who were kidnapped last month, but that it is still too early to begin planning a rescue mission.
The U.S. has 16 people at its embassy in Abuja, the capital, who are assisting the Nigerian military with the search. It is providing both "manned and unmanned" intelligence resources to the government, according to Hagel.
"Intelligence is critical here and we're devoting all our capabilities" to that, he said.
But Hagel indicated that the girls must be located in order to build a plan to get them back. Although U.S. once sent 100 soldiers to aid in the hunt for warlord Joseph Kony, the defense secretary said that might not be an effective strategy in this instance.
"We have to assess first of all intelligence on what we have, what we know, where are they," he said. "This is a vast, vast area, and it's heavy with jungle...probably as dense as any in the world. Other nations border Nigeria. It could well be that some of these girls are in some of these other countries; we just don't know enough yet to be able to assess what we will recommend to the Nigerians," he said.
He admitted it's an "open question" whether the Nigerian military would be capable of a major hostage rescue. While the U.S. will help in every way possible, they still have to respect Nigeria's sovereignty, Hagel said, noting that the U.S. had to wait to be invited to help.
State Department Africa specialist Robert Jackson told a Senate Foreign Relations subcommittee Thursday morning that resolving the crisis has become "one of the highest priorities of the U.S. government."
The attack by Boko Haram, the Islamist group that is responsible for the kidnappings,is part of a "long, terrible trend" of killing people in Nigeria, Jackson said.
He was also pressed on whether the U.S. should have acted sooner to label Boko Haram a terrorist organization, which it declined to do in the past because the Nigerian government feared it would bring the group infamy and publicity.
"We might have done it earlier. I think the important thing is that we have done it and that we've offered a reward for the top leadership of Boko Haram's location," he said.
Alice Friend, the Defense Department's principal director for Africa, conceded at the hearing that Nigeria's own human rights problems have hampered the search because of a U.S. law that restricts the U.S. from providing assistance to foreign military units suspected of gross human rights violations.
Friend also said that the Nigerian government "has failed to mount an effective campaign against Boko Haram," and the extremist organization has become more capable over time. The Nigerian military, by comparison, has been slower to adapt new strategies and tactics.