"We are actively advising -- including through military personnel -- the Nigerian government as it seeks to find, to locate and to rescue these girls. Finding them is the first step," White House Press Secretary Jay Carney said.
"Our military personnel at the embassy and any additional military personnel we may deploy will be Nigeria in an advisory capacity," he said.
More than 270 girls were snatched by militants from their boarding school in Chibok, north-eastern Nigeria, on April 14. Some managed to escape, but most were taken into the remote Sambisa forest.
Carney said: "I think it's important to be mindful of where we are in this process and not get ahead of ourselves. We're engaged at many levels now as part of this group that's been stood up at the embassy with personnel from the embassy as well as from AFRICOM and elsewhere, including military, State and FBI to assist the government in the effort underway to find the kidnapped girls."
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"I would note that even the narrowly-drawn area where there is the greatest suspicion that the girls might be is an area that is something along the size of the state of West Virginia. So this is a pretty vast expanse of territory," he said.
Carney said the US does not have intelligence to share about the location of the girls or whether they're all together.
"But time is of the essence, as we've been saying. That's why we were so eager to assist the Nigerian government in this effort that the Nigerian government, of course, is leading. But it's important to step back and understand the challenges that this search effort is facing," he said.
"We would have to know where the girls are as a simple proposition before we could attempt rescue attempts. At this point, we're not considering the deployment of US military personnel. The military personnel who are involved in this effort are involved in an advisory capacity," he said.