The United States "could become directly involved in a military conflict" between Tokyo and Beijing over the Senkaku Islands, the Congressional Research Service said in a report.

Its report on Japan-U.S. relations said that "China has conducted increasingly aggressive operations by dispatching both military and maritime law enforcement ships and aircraft to the area" since Tokyo bought part of the islets from a private owner last September.

China's locking of weapons-targeting radar on a destroyer near the islets was "considered a major escalation in the standoff," the congressional think tank said.

The incident also "sparked questions about whether the Chinese operator was acting on orders from Beijing, military commanders, or his own discretion," it said. China has denied the use of weapons-targeting radar.