North Korea has built two frigates capable of carrying helicopters, its largest weapons system in 25 years, a military analyst said, raising questions over the effectiveness of sanctions aimed at deterring its arms buildup.

The vessels are equipped with rocket launchers and what look to be anti-ship and anti-aircraft missile mounts, Joseph S. Bermudez Jr., chief analytical officer at AllSource Analysis, wrote on 38 North, a blog run by the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies in Washington. Bermudez based his conclusion on commercial DigitalGlobe images from December and January.

The frigates “appear to be primarily designed to counter what Pyongyang sees as a growing threat from South Korea’s acquisition of submarines that began in the early 1990s,” he said in the posting yesterday. “These vessels may have an important secondary role: the protection of fisheries in the West and East Seas.”

Since 1999, North and South Korean naval vessels have engaged in three fatal skirmishes near their disputed western sea border. In 2010, 46 South Korean sailors died in an attack on their ship that Seoul blames on a North Korean submarine.

The North may reach out to China or Iran to help improve the frigates’ radar, sonar, electronic and anti-air capabilities, said Bermudez, who also operates KPA Journal, a website dedicated to analyzing the North’s military.

“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,” he said.

North Korea, which threatened earlier this year to detonate a fourth nuclear device, was hit with United Nations sanctions after tests in 2006, 2009 and 2013.

The construction of the frigates began between 2006 and 2007 and it is unclear whether they are ready for service, Bermudez wrote. Kim Jong Un’s regime has a 60,000-man navy with 420 warships and 260 landing vessels, South Korea said in its 2012 defense analysis.

The two Koreas remain technically at war as the truce that ended the 1950-53 conflict hasn’t been replaced with a peace treaty. They have held only two rounds of official high-level talks since 2007.

On May 8, South Korea accused North Korea of sending two drones found crashed near the border to spy on the country. The North denied sending them.