North Korea is gearing up to declare an all-out war on its rival South Korea in order to unify the two countries by next year, Seoul has alleged.

By boosting its military capabilities and doubling its forces in several units, North Korea could be planning a full-fledged war, South Korea said.

"After declaring 2015 the year of completing unification, North Korea has been prepared for full-scale wars. For that goal, the North doubled the number [of service-members] involved in its summer trainings compared to previous years. It has also boosted its attack capabilities in a consistent manner," South Korea's defence ministry said in a document submitted to the country's national assembly, according to the Yonhap news agency.

On at least 19 occasions this year, North Korea has fired rockets targeting South Korean territories keeping up tensions high in the Korean peninsula.

Seoul has, however, said they are prepared to deal with any eventuality.

The defence ministry added: "South Korea has maintained a firm readiness against North Korea's limited provocations and any attempts to wage full-scale wars. We are trying to boost security posture in capital areas to deter diverse and novel types of threats by the enemy."

The latest threat perception of South Korea comes at a time when ships belonging to the two Koreas have exchanged warning shots on a disputed maritime border.

The brief confrontation which lasted for about 10 minutes took place at about 09:50am local time after a North Korean patrol boat crossed what South Korea calls the Northern Limit Line (NLL).

South Korea's Joint Chiefs of Staff said in a statement: "To force the vessel to retreat, our side issued warning messages and fired a warning shot. But the North Korean vessel fired back rather than backing down, which caused us to fire again. Then the ship made a retreat."

"We have maintained full readiness while closely watching the North Korean military's movements. No other extraordinary movements were detected at the border region when the North's vessels intruded onto our side."