By Phil Stewart
WASHINGTON, June 27 (Reuters) - The United States reduced its surveillance flights to help find more than 200 Nigerian schoolgirls abducted by Islamist militants after building a body of intelligence and after other states ramped up support, a U.S. official said.
Nigeria has committed itself to the hunt for the girls, who were kidnapped in April in one of the violent group's most spectacular attacks, and received help from the United States and other countries, including its neighbors.
The senior U.S. defense official told Reuters that the U.S. intelligence, surveillance and reconnaissance flights, first announced in May, were now flying at an "intermittent" rate.
The official said overall intelligence-gathering had not diminished, and noted additional operations by Britain and France.
"We had substantial initial coverage for the baseline and we've moved into a maintenance mode," the official said, speaking on condition of anonymity.
The official declined to say how long the period of heightened initial U.S. coverage lasted. Asked whether it was just a week or two, the official said: "No. We were ... building this baseline for a good period of time."
The Pentagon had said on Thursday that there were "around the clock" intelligence, surveillance and reconnaissance (ISR) operations in support of Nigeria's search. U.S. military personnel are in Abuja helping coordinate the effort.
The United States also sent about 80 U.S. military personnel to Chad in May to support the surveillance operation. Chad lies to the northeast of Nigeria, bordering the area in which Boko Haram operates.
In the last month U.S. officials have played down expectations about a swift rescue of the girls and stressed the limitations of intelligence gleaned from surveillance flights.
One U.S. official told Reuters of concerns that Boko Haram may have laid booby traps in areas the girls could be held and there have been reports that the girls may have been split up into small groups.
"ISR alone will not solve this problem. It will take the Nigerian piece of the equation with their own sources and human intelligence coupled with the other forms to really understand the picture," the defense official said.
In an opinion piece in the Washington Post on Friday Nigerian President Goodluck Jonathan said his government and security services had "spared no resources, have not stopped and will not stop until the girls are returned home."
The defense official did not discuss specific U.S. intelligence but acknowledged that information gathered from different sources had left only a murky picture of where the girls might be, in how many groups and even in which country.
"What is clear is a sense of dispersion that would contribute to pessimism in terms of the prospects for a successful rescue operation to be mounted by anyone, whether it's the host nation or supported in any way by external actors," the official said.
(Additional reporting by Susan Heavey; Editing by David Storey and Sofina Mirza-Reid)