A senior U.S. defense official says the presence of large numbers of Chinese ships near islands and islets occupied by the Philippines is "a concern."

Joseph Felter, deputy assistant secretary of defense for South and Southeast Asia, said Friday that the Chinese activities were "somewhat aggressive and provocative." He was asked about the issue while on a visit to Bangkok.

The Philippine government protested the movements Thursday after its military monitored more than 200 Chinese vessels from January-March in a disputed area named Sandy Cay, which is near the Philippine-occupied island of Pag-asa.

Felter noted that the U.S. does not take a stance on various conflicting claims in the South China Sea. But he said the U.S. would work with allies and partners to keep the seas free and open.