SEOUL (AFP) - North Korea appears to be making preparations to conduct its fourth nuclear test, a US think-tank said on Friday just hours before President Barack Obama arrives in South Korea for a visit.

New satellite images, taken just two days ago, showed additional activity at the Punggye-ri test site that is "probably related to preparations for a detonation", the US-Korea Institute at Johns Hopkins University said.

The report echoed recent warnings from South Korea that the North might be planning a test to coincide with Obama's two-day visit.

Analysis suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels, the institute said on its closely followed 38 North website.

Also visible were probable command and control vehicles intended to provide secure communications between the test site and other facilities.

North Korea has conducted three nuclear tests in 2006, 2009 and 2013.

The 38 North analysis noted that preparations for the test in February last year had peaked two or three days before detonation.

The withdrawal of all equipment, vehicles and personnel had occurred immediately before the blast.

"Whether North Korea will follow the same timeline in 2014 remains unclear," it said.