The Obama administration on Tuesday underscored its opposition to any offer of ransom or other concessions to retrieve more than 250 schoolgirls abducted by the terrorist group Boko Haram, even as the United States widened its participation in the international effort to locate and free the girls.

“We, as a matter of policy, deny kidnappers the benefits of their criminal acts, and that includes ransoms or other concessions,” White House press secretary Jay Carney said. He said the Nigerian government is leading the search effort, however, and the United States is merely assisting.

That was a tacit acknowledgment that despite the outpouring of American popular and political support for efforts to free the girls, the United States cannot intervene if Nigeria chooses to pay off the terrorist group or release captured militants in a trade.

The government of Nigerian President Goodluck Jonathan said Tuesday that the “window of negotiation” is open with Boko Haram rebels who snatched the girls a month ago from their boarding school in the northeast of the country.

The rebel group’s leader said Monday the girls would not be seen again unless the Nigerian government releases “our brethren.”

A member of Jonathan’s cabinet, Minister of Special Duties Tanimu Turaki, told the Reuters news agency that the government has established a committee to negotiate with Boko Haram. The willingness to barter suggested that the government may be reconsidering its previous stated opposition to a prisoner release or the payment of ransom.

Boko Haram was paid a reported $3.1 million last year for the release of a French family taken hostage. The release was arranged by French and Cameroonian negotiators, but it was not clear who paid the ransom.

The payment of ransom is fairly routine for other governments, including many closely allied with the United States. U.S. opposition to the practice is primarily based on a view that such negotiation encourages more kidnappings and ever larger payouts.

French Foreign Minister Laurent Fabius told reporters in Washington on Tuesday that France is again willing to help, saying his government is one of the few with contacts and experience in the remote, lightly populated area of northern Nigeria and neighboring Cameroon.

“This is a mass rape that has to be investigated and prosecuted as such,” Fabius said of the abduction. He spoke in French through an interpreter.

France has invited the leaders of Nigeria and Cameroon, along with other African leaders, to a strategy session with European and U.S. diplomats this weekend in Paris. It was not clear whether Secretary of State John F. Kerry would attend. Fabius and Kerry met at the State Department and did not address the Nigeria case during brief public remarks.

The girls’ plight received little international attention at first, and the girls’ families and supporters contend that they got little or no help from the Nigerian government. International pressure on the Jonathan government increased rapidly over the past two weeks, along with news media and social media attention to the abduction.

The United States is contributing a manned surveillance plane, commercial satellite data and other help. A team of 27 U.S. experts and security personnel is in place in the Nigerian capital, Abuja, State Department spokeswoman Jen Psaki said Tuesday.

Psaki said the United States, which blacklisted Boko Haram as a terrorist group last year, is also seeking U.N. sanctions on the group.