President Ferdinand Marcos Jr. said on Monday the presence of a Russian attack submarine in the Philippines' exclusive economic zone (EEZ) in the South China Sea was "very worrisome". "That's very concerning.

Tensions between Russia and the Philippines escalated as the Philippine military deployed a navy ship and air force planes to shadow a Russian submarine in the South China Sea last week, security officials said.

Officials say the Russian vessel was not in Philippine territorial waters during its maritime exercise with the Malaysian navy.