A North Korean long-range rocket launch appears unlikely at the upcoming 70th anniversary of the ruling party, a U.S. research institute said Thursday.

Pyongyang is expected to stage its biggest celebration in years on Oct. 10, and there's speculation it could launch a rocket to advance its fledgling space program. The U.S. views that as a way for the North to develop a long-range ballistic missile.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies is basing its analysis on recent commercial satellite photos, the latest taken Monday, of the North's main rocket launch site at Sohae. Research being published on the institute's website, 38 North, says a launch by Oct. 10 is still possible but there's no sign of preparations that would have been expected by now for that to happen.

The institute does detect new activity at North Korea's nuclear test site at Punggye-ri but says its purpose is unclear.

Satellite imagery from Sept. 17 shows four large vehicles parked near a tunnel entrance, which is concealed by camouflage netting, often used to hide activity from overhead scrutiny.

Predicting nuclear tests, which are conducted underground, is notoriously difficult. The institute says movement of vehicles and other activity at the site could indicate anything from maintenance work to preparations for North Korea's fourth test explosion since 2006.

North Korea fired its first rocket into space from Sohae on its west coast in December 2012, sending into orbit a satellite that soon malfunctioned. It then conducted a nuclear test at Punggye-ri in February 2013. Both actions drew U.N. Security Council condemnation and sanctions.

Pyongyang has said it has the right to launch rockets any time it sees fit but has not said when it would do so. Last week, it announced it has upgraded and restarted all of its atomic fuel plants — meaning it could possibly make more, and more sophisticated nuclear weapons.

Jeong Joon-Hee, a spokesman for South Korea's Unification Ministry, told reporters Friday that there were no signs that a North Korean rocket launch is imminent.