Satellite images of a North Korean rocket launch site show a mobile radar trailer and rows of what appear to be empty fuel and oxidiser tanks, evidence of ramped-up preparation for what Washington calls a cover for a long-range missile test.

An analysis of images that the US-Korea Institute at Johns Hopkins School of Advanced International Studies gave to Associated Press on Monday shows Pyongyang "has undertaken more extensive preparations for its planned April rocket launch than previously understood". The images were taken on Wednesday.

A mobile radar trailer essential for any launch stands at the end of a new dirt road running from the entrance of the Tongchang-ri site; it has a dish antenna that is probably a radar tracking system, according to the institute's analysis. Radar tracking during a launch gives engineers crucial real-time information on the performance of the rocket's engines, guidance system and other details.

"These pictures are new and important evidence that the north's preparations for its rocket launch are progressing according to schedule," said Joel Wit, visiting fellow at the institute and editor of its website on North Korea, 38 North. The images are from DigitalGlobe, a commercial satellite photography company.

North Korea says the launch, set for some time between 12 and 16 April, will fire a satellite into orbit to study the country's crops and natural resources. It is also meant to honour what the Pyongyang regime sees as one of the country's most important days – the centenary of the 15 April birth of national founder Kim Il-sung.

Washington says North Korea uses such launches to test missile systems for nuclear weapons that could target the US. While North Korea has conducted two nuclear tests, analysts do not believe it has yet mastered the technology needed to shrink a nuclear weapon and mount it on a missile.

Any launch would signal the end to the 29 February accord between North Korea and the US that would lead to US food aid exchanged for a North Korean moratorium on missile and nuclear tests, as well as a suspension of nuclear work at the main Yongbyon nuclear facility. The US says plans to ship food are already on hold.

The launch would be the fourth of its kind since 1998, when Pyongyang sent a long-range rocket hurtling over Japan. The last rocket launch, in 2009, led to UN condemnation and the north walking away from six-nation nuclear disarmament talks; weeks later, Pyongyang carried out its second nuclear test.

The planned launch could demonstrate if North Korea is closer to perfecting a multi-stage rocket that could hit the US. Analysts fear a launch could spur a chain of events that would mirror 2009 and send tensions soaring again on the Korean peninsula. A year after the last test, 50 South Koreans were killed in attacks blamed on North Korea.

The storage area at the Tongchang-ri launch site. Photograph: AP

The satellite images show what are likely to be empty fuel and oxidiser tanks in previously empty, fenced-in areas, the institute's analysis says.

"The tanks were apparently dumped in these locations after their contents were transferred to buildings that will directly fuel the first stage of the Unha-3 [rocket]," according to the analysis. "The large number of apparently empty tanks indicates that the transfer process may have been close to completion."

The announcement of the latest launch came just two weeks after the US-North Korean nuclear freeze agreement, which had buoyed hopes for improved relations between the wartime enemies under new North Korean leader Kim Jong-un. He came to power after his father Kim Jong-il died of a heart attack in December.

North Korea's ruling party announced on Monday it would hold an important political conference on 11 April in Pyongyang. Kim Jong-un is expected to gain new titles at the conference, which comes shortly before the planned launch.

The Tongchang-ri rocket launch site is about 35 miles from the Chinese border city of Dandong. North Korea has said that the southerly flight path from the site was chosen so debris would not hurt neighbouring countries.

But that has not assuaged widespread fear over falling debris from the rocket. Japan's defence minister has ordered missile units to intercept the rocket if it or its fragments threaten to hit Japan. South Korea has also warned it might shoot down any parts of the North Korean rocket heading for South Korean territory.

South Korean defence officials have said the main body of the three-stage rocket was transported to a building in Tongchang-ri.