The Finnish military says it has dropped depth charges onto a suspected submarine in the sea outside Helsinki after twice detecting the presence of a foreign object in the area.

The navy's maritime operations chief, Olavi Jantunen, says they could not identify the "underwater object" but had begun an investigation, which could take weeks.

Parliamentary defense committee spokesman Jussi Niinisto said Tuesday that the military occasionally detects such activity but described the use of depth charges as unusual.

The intrusion comes in the wake of a lengthy hunt for a foreign submarine in Swedish waters in October following increased military activity in the Baltic Sea region.

Swedish officials never blamed any country though most defense analysts said Russia was a likely culprit.