The U.S. National Security Agency sought the Japanese government’s cooperation in 2011 over wiretapping fiber-optic cables carrying phone and Internet data across the Asia-Pacific region, but the request was rejected, sources said Saturday.

The agency’s overture was apparently aimed at gathering information on China given that Japan is at the heart of optical cables that connect various parts of the region. But Tokyo turned down the proposal, citing legal restrictions and a shortage of personnel, the sources said.

The NSA asked Tokyo if it could intercept personal information from communication data passing through Japan via cables connecting it, China and other regional areas, including Internet activity and phone calls, they said.

Faced with China’s growing presence in the cyberworld and the need to bolster information about international terrorists, the United States may have been looking into whether Japan, its top regional ally, could offer help similar to that provided by Britain, according to the sources.

Based on documents leaked by former NSA contractor Edward Snowden, British newspaper The Guardian reported that the agency had been sharing data intercepted by Britain’s spy agency, GCHQ, through transatlantic cables since 2011.

But Tokyo decided it could not do so because under current legislation, it cannot intercept such communications even if the aim is to prevent a terrorist act. Japan also has a substantially smaller number of intelligence personnel, compared with the NSA’s estimated 30,000 employees, the sources said.

A separate source familiar with intelligence activities of major nations said the volume of data that would need to be intercepted from fiber-optic cables would require a massive number of workers and the assistance of the private sector.