Several Chinese vessels have been deployed near the disputed Scarborough Shoal in the South China Sea close to the Philippine coast, fueling further tensions in the region, the New York Times reported Monday citing Philippine Defense Minister Delfin Lorenzana.

Both Beijing and Manila have laid claims to the Scarborough Shoal that is considered an important strategic zone due to its proximity to military bases in the Philippines, a U.S. ally.

According to Lorenzana, the Philippine Defense Department has pictures of four Chinese Coast Guard ships, and six other vessels, seen less than a mile from the shoal. There are chances that those vessels could be dredges to carry out initial building work, he told the Times.

In recent weeks, four Chinese Coast Guard ships have been permanently deployed close to the shoal, Lorenzana said, adding that the six other vessels were the latest additions seen by Filipino fishermen last week. Following this, a Philippine Navy plane was sent to photograph the vessels, of which six appeared to be civilian ships but could be navy ships “masquerading” as other ships, Lorenzana told the newspaper.

The defense minister said that the Chinese ambassador in Manila was warned about the vessels last week. However, the ambassador denied that any Chinese ships were close to the shoal, according to Lorenzana.

China, Vietnam, the Philippines, Taiwan, Malaysia and Brunei all have competing claims over territory in the South China Sea. About $5 trillion worth of maritime trade passes through the South China Sea each year. China has been reportedly building runways and ports on islands in South China Sea to assert its claim over the region.

China’s assertive nature in the disputed region has drawn criticism from its neighbors and the United States. However, Beijing has consistently defended its actions, saying it does not have any intention of starting a conflict and that its operations will add to the safety of the region.