North Korea is unlikely to conduct another nuclear test at least until this fall, a U.S. think tank said Friday, citing satellite imagery showing no signs of preparations at the country's underground test site.

"Recent commercial satellite imagery indicates that North Korea is conducting regular spring construction and maintenance activities at its Punggye-ri nuclear test site," the website 38 North said in a report by analyst Jack Liu.

"There are no indications of nuclear test preparations at this time. Given the time and effort such preparations require, North Korea is unlikely to conduct another nuclear test until at least fall 2015 at the earliest," the report said.

The North has conducted its three nuclear tests in 2006, 2009 and 2013, all at the Punggye-ri site in the country's northeast.

Speculation mounted in early 2014 that the North could conduct its fourth nuclear test after the regime threatened to conduct a "new form of nuclear test."

In November, Pyongyang again threatened to test a nuclear device after the Third Committee of the U.N. General Assembly passed a resolution that called for referring the North to the International Criminal Court for human rights violations. (Yonhap)