The U.S. Defense Department on Tuesday informed lawmakers about a possible sale to Taiwan of Patriot missile system upgrades valued at $939 million and supplied by U.S. defense contractor Raytheon Co.

The Pentagon's Defense Security and Cooperation Agency said the government of Taiwan had requested upgrades and refurbishment of its three existing Patriot fire units.

Congress has 30 days to block the sale although such action is rare.

The agency said the proposed sale served U.S. national, economic and security interests by supporting Taiwan's continuing drive to modernize its armed forces and boost its defense capability.

The sale, likely to irritate China, comes just a week after Defense Secretary Robert Gates appealed to China for help in curbing Iran's nuclear program, arguing that a stable Gulf was in the interests of Beijing's energy security.

Ties between the United States and China are much warmer than earlier this decade, but Washington remains concerned about China's military modernization and Beijing's decision to shoot down one of its own aging satellites last January.

During his visit to Beijing, Gates said China had not adequately explained the anti-satellite test, a move that raised the specter of a space arms race. China says its space programs are entirely for peaceful purposes.

At the time, Gates said he had reiterated Washington's position on Taiwan, the self-governed island over which China claims sovereignty, saying it opposed moves by either side to change the status quo.

The defense agency said the proposed sale included an array of long- and short-range radio systems, radar enhancements, remote launch communications equipment and an electric power plant, as well as four telemetry kits for live fire training.