The Project on Government Oversight, which has been warning about vulnerability to terrorists, said a briefing prepared by the Government Accountability Office indicated that five of the 11 sites will miss improvement deadlines, The New York Times reported Monday.

A copy of the briefing materials prepared for the U.S. Senate was provided to the Times by the watchdog group.

One site that will miss its deadline by years, the briefing materials claimed, is the Oak Ridge National Laboratory in Tennessee that will take until 2015 to dilute its large stock of weapons-grade uranium.

Two other sites set to miss deadlines are operated by the National Nuclear Security Administration whose director was forced to resign in January because of security lapses, the Times said.

"The department seems to think that the terrorist threat to its nuclear facilities is no more serious than a Halloween prank," U.S. Rep. Edward Markey, D-Mass., told the newspaper.