North Korea has built two tunnel entrances at its nuclear test site in a sign it plans more of its internationally condemned detonations, a US think tank said Wednesday.

Satellite images taken on September 27 shows progress in excavation work and the presence of two new entrances to tunnels at the Punggye-ri site, Johns Hopkins University's US-Korea Institute said.

The research group said it had no reason to suspect an imminent test but that the work showed clear intentions to advance North Korea's nuclear program.

"These ongoing activities as well as upgrades to the site's support areas indicate North Korea is preparing to conduct additional detonations in the future as part of its nuclear weapons development program," researcher Nick Hansen wrote on the institute's blog, 38 North.

North Korea, which conducts tests in secrecy underground, may be seeking a new tunnel for a future detonation, it said.

Alternatively, the reclusive regime may be digging a separate entrance to a nearby tunnel to allow greater traffic or ventilation.

North Korea has carried out nuclear tests in 2006, 2009 and in February this year, triggering virtually universal criticism from major countries and tight sanctions.

Kim Jong-Un's regime has said it is building its nuclear capacity in response to "hostility" from the United States, which stations close to 30,000 troops in the North's democratic rival South Korea.

US researchers have also observed progress at the aging plutonium reactor at Yongbyon, which would allow North Korea to expand its nuclear program.