NTT Docomo Inc. is set to terminate its mobile phone service for Japanese people in the United States next year in a move that would mark the effective withdrawal of Japan's largest mobile carrier from the giant market.

It would also highlight the urgent task Docomo faces in restructuring its strategy for overseas expansion, after it announced last April plans to pull out of India.

Docomo is mainly providing phone and data services to Japanese expatriates and students in New York, Washington, Los Angeles and Hawaii. It will end the operations at the end of March 2016, the industry source said Wednesday.

Docomo had withdrawn from the U.S. market in 2004 but re-entered in 2011 in a partnership with T-Mobile USA Inc. in 2013 aimed at improving its network services for the hundreds of thousands of Japanese living in the United States.

Amid fierce competition, however, Docomo has been struggling to secure subscribers and its now looking to slash its 70 U.S. staff by 40 percent while keeping some in Washington to collect business information, the source said. It will continue to provide a roaming service for Docomo handsets in the United States.