New satellite images suggest that North Korea may be in advanced stage of developing a submarine-launched ballistic missile, a U.S. think tank said Tuesday. Pyongyang’s active pursuance of the missile comes at a time of heightened tensions in the Korean Peninsula, where the reclusive state has carried out several nuclear and missile tests despite stern sanctions.

An April 28 satellite imagery of North Korea’s Sinpo South Shipyard shows a submersible missile test stand barge has been moved from its normal position, 38 North, a project of the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies, said. The activity was seen six days after the country tested a submarine-launched ballistic missile.

“This shift in location suggests that the SLBM [submarine-launched ballistic missile] program may be moving towards a more advanced stage of development where the need for a submersible test stand barge has diminished,” 38 North said. “That conclusion appears to be reinforced by recent test launches from the submarine itself rather than the barge,” it said, adding that the recent SLBM launch was “successful.”

“While North Korea’s submarine-launched ballistic missile program is making progress, these activities appear in line with previous estimates that the first ballistic missile submarine and operational missiles are unlikely to become operational before 2020,” according to the North Korea-monitoring project.

North Korea, which is gearing up for the key meeting of its ruling Workers’ Party Friday, is expected to conduct its fifth nuclear test around that time, South Korean defense ministry said Monday. The Kim Jong Un-led regime has conducted a series of missile and nuclear tests since the beginning of this year, and the leader has called to strengthen the country’s defense capabilities. Pyongyang has also claimed to have secured intercontinental ballistic missile technologies like reentry and engine technologies.