U.S. defense company Raytheon has been lobbying lawmakers in Japan to replace Lockheed Martin Corp. as the supplier of powerful radar systems, as Tokyo reconsiders plans for two Aegis Ashore missile defense sites, three sources have said.

"It's game on," said one of the sources, who has direct knowledge of Raytheon's lobbying campaign. Raytheon's pitch includes a proposal to put its SPY-6 radar on refitted destroyers, as the U.S. Navy plans to do. The company says that would save money and time as Japan tackles new missile threats, drones and stealth aircraft.

Lockheed Martin has a contract with Japan to build its $300 million SPY-7 radars at the two canceled Aegis Ashore sites, and says installing the systems at other sites or aboard ships are both possibilities.