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. The navy said it noticed an underwater target yesterday and again this morning and fired some warning charges the size of grenades. Finland, which shares an 833-mile  border with Russia, has been increasingly worried about its powerful neighbour after a year of Russian air force sorties and military border exercises. A Finnish coastguard ship tracks the underwater object - believed to have been a Russian submarine - in the waters near the capital Helsinki . Border patrol boats first identified an underwater target yesterday, before detecting it again early this morning and dropping 'warning' depth charges . Finland defence minister Carl Haglund did not say whether Russia was involved but told local media that it was extremely rare for the military to use such warning charges. Pictured is a Finnish navy boat . The Finland incident comes just months after Sweden's armed forces hunted unsuccessfully for what they believed to have been a foreign submarine close to Stockholm . It comes just months after Sweden suspected Russia of sending a vessel into waters close to the capital Stockholm. In what was Sweden's biggest mobilisation since the Cold War, its navy hunted unsuccessfully for a week for what they believed to be a foreign submarine after several observations were made. Swedish officials never blamed any country, though most defense analysts said Russia was a likely culprit. Today Finland defence minister Carl Haglund did not say whether Russia was involved but told local media that the target could have been a submarine, and that it has likely left the area, adding that Finland has rarely used such warning charges. He said: 'We strongly suspect that there has been underwater activity that does not belong there. Finland first alerted its navy to a 'possible underwater target' about midday yesterday . 'Of course it is always serious if our territorial waters have been violated,' he told Finnish news agency STT. Moscow retorted immediately, saying moves by Finland and Sweden towards closer ties with NATO were of 'special concern'. In a statement, the Finland Ministry of Defence said its surveillance system first alerted its navy to a 'possible underwater target' within territorial waters about midday yesterday. A second detection was then made early this morning after the navy began searching for the object, and underwater depth charges were fired at 3am. Commodore Olavi Jantunen told Helsingin Sanomat newspaper: 'The bombs are not intended to damage the target, the purpose is to let the target know that it has been noticed.' Reports of a submarine spotted off Stockholm last year led to Sweden's biggest mobilization since the Cold War. Regional tensions were reflected earlier in April after an unprecedented hawkish joint statement by Nordic countries - Sweden, Norway, Finland, Denmark and Iceland - that directly cited the Russian 'challenge' as grounds to increase defense cooperation.
The Finnish military has dropped depth charges on a possible submarine.
Its navy twice detected a foreign object within Helsinki territorial waters.
The charges released were warning shots, about the size of hand grenades.
Experts believe the object is likely to have been a Russian submarine.