Satellite images have picked out two new North Korean warships -- the largest it has constructed in 25 years and an important "wake-up call" on the effectiveness of sanctions, a US think-tank said Friday.

Recent commercial satellite pictures showed two new helicopter-carrying frigates separately berthed at shipyards in Nampo in the west and Najin in the far northeast.

Launched sometime in 2011-12, the two vessels were primarily designed to counter what Pyongyang sees as a growing threat from South Korea's acquisition of submarines that began in the early 1990s, the US-Korea Institute at Johns Hopkins University said.

But they may also be destined for a role in patrolling regional fishing zones -- with security implications for South Korea, Japan and China, the institute said in an analysis on its website 38 North.

While it might still take several years to fully integrate the frigates into fleet operations, the institute said their introduction suggested an "evolutionary step" in the North's naval strategy to include helicopter anti-submarine operations.

The analysis also noted that the construction of the two warships and other new naval classes had been achieved during a period of prolonged international economic sanctions.

"North Korea's deployment of new helicopter frigates may be an important wakeup call about the overall effectiveness of sanctions and the need to carefully and realistically reevaluate reports of its conventional military decline," it said.

The development of the North's conventional weaponry has largely been overshadowed by concerns over its ballistic missile and nuclear weapons programmes.