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.”

Axar.az reports that citing Reuters, “That’s very concerning. Any intrusion into the West Philippine Sea, of our EEZ, of our baselines, is very worrisome,” Marcos told reporters, referring to part of the South China Sea within the Philippines’ maritime zone.

A Russian Kilo-class submarine was sighted 80 nautical miles off the western province of Occidental Mindoro on Nov. 28, Navy spokesperson Roy Vincent Trinidad said in a statement on Monday.

“The Russian vessel stated it was awaiting improved weather conditions before proceeding to Vladivostok, Russia,” Trinidad said, without elaborating on why it was in the area.

Russia’s Kilo-class submarines are considered some of the quietest submarines and have been constantly refined since the 1980s.