SEOUL (AFP) — South Korea has voiced concerns to China over its establishment of a “no-sail zone” in a shared area of the Yellow Sea, Seoul’s foreign ministry said Saturday.

The United States outlet Newsweek reported Wednesday that China had barred ships from entering certain parts of the Yellow Sea, located between mainland China and the Korean Peninsula.

According to Newsweek, a regional branch of China’s Maritime Safety Administration (MSA) declared the no-sail zone in parts of the sea’s Provisional Measures Zone (PMZ) — a shared area where the two countries’ exclusive economic zones overlap — until 27 May.

The MSA did not provide a reason, according to Newsweek, but South Korean media reported it was for “military training purposes,” citing the Korea Hydrographic and Oceanographic Agency.

Seoul’s foreign ministry said Saturday that both countries were allowed to conduct military drills in the shared zone, but that “China’s establishment of a no-sail zone within the PMZ that excessively restricts freedom of navigation raises concerns.”

Accordingly, Seoul has “conveyed such concerns to the Chinese side through diplomatic channels,” it added.

The ministry also noted it was coordinating with other agencies, including the defense ministry, to “assess whether China’s recent measure is in accordance with international maritime law.”

According to Newsweek, Beijing has also installed three structures which it claims are “aquaculture facilities.”

Some South Korean media outlets have suggested that these actions within the PMZ could be “grey zone” tactics, deliberately ambiguous moves aimed at expanding China’s maritime jurisdiction.