China has conducted several anti-ship ballistic missile tests in the South China Sea , two US officials told NBC News Monday.

According to the unidentified officials, the first test was conducted over the weekend when at least one missile was fired off. With the window for official testing open until July 3, officials expect the Chinese military to fire other missiles into the sea before then. 

The officials also noted that the US military ships in the South China Seawere not in danger. However, one of the anonymous officials said that the test is “concerning.” The official refused to comment on whether the anti-ship missiles being tested “represent a new capability for the Chinese military,” NBC News reported.

The South China Sea contains many islands, reefs and shoals presently controlled and occupied by the People’s Republic of China but also claimed by several other nations, including Vietnam, Taiwan, Cambodia, Brunei and the Philippines. Aside from the enormous amount of international sea trade that passes through the area, it is also believed to contain large amounts of as-yet unexplored oil and natural gas deposits. 

The United States and its allies, such as the UK, France, Canada, Australia and Japan, have repeatedly challenged China’s territorial and maritime claims by conducting so-called freedom of navigation operations, sailing military vessels in the South China Sea . This week, France and the UK earlier promised to send their warships to the waters claimed by China, Sputnik reported. 

News of China’s drills come after US President Donald Trump met Chinese President Xi Jinping during the G20 summit in Osaka, Japan, this weekend, and Trump agreed not to increase the already existing US tariffs against China, noting that the trade negotiations would continue.

In May 2018, China secretly installed anti-ship cruise missiles and surface-to-air missiles in three South China Sea outposts. The land-based defensive missile systems were set up in Fiery Cross Reef, Subi Reef and Mischief Reef in the Spratly Islands, anonymous sources told CNBC at the time.