Already under fire for its latest nuclear test, North Korea has been making preparations at a launchpad that could pave the way for firing a long-range missile, a US think tank said Thursday.

38 North, a blog of the US-Korea Institute at Johns Hopkins University, analysed satellite photos that it said showed possible assistance from Iran at the Tonghae Satellite Launching Ground in northeastern North Korea.

38 North said that a commercial satellite photograph taken on January 5 showed "important progress" since the area was hit with typhoons last year and indicated activity that was consistent with preparations for a launch.

But the website said there was not enough evidence to support speculation that North Korea could raise the stakes by testing its KN-08, a new missile with potential intercontinental range first displayed in a parade in April.

The website said that North Korea likely was preparing to test much larger rockets by the time the site is completed in 2016. The launchpad was last used in April 2009 for a long-range test widely seen as a failure.

The area has a new flame trench covering, which would protect large rockets from exhaust gases, that has a design similar to one used at the Semnan launchpad in central Iran, the website said.

It said that the satellite photos showed that three storage tanks were built last year that can hold 439,100 liters (116,000 gallons) of fuel, a level far above North Korea's capacity during its most recent launch.

38 North said that it also observed a conduit through which technicians and electrical and communications lines can pass, even though Tonghae is accessible only by a dirt road.

North Korea defiantly carried out its third nuclear test on Tuesday, saying it was responding to US "hostility" after the UN Security Council condemned its last launch two months earlier.

On December 12, North Korea launched a rocket from its separate Sohae site that, according to experts, managed to reach near the Philippines and to put a small satellite into orbit.

The United States and its allies condemned the launch, fearing it was part of efforts to refine a longer-range missile.

Meanwhile more than 100,000 troops and civilians staged a mass rally in Pyongyang to celebrate North Korea's nuclear test and praise the "matchless" bravery of leader Kim Jong-Un, state media said Friday.

The rally in the capital's sprawling Kim Il-Sung square on Thursday was attended by top party and military officials, as well as police workers and students, the official Korean Central News Agency (KCNA) said.

A number of speakers addressed the rally, praising Tuesday's test as the "brilliant fruition of the extraordinary decision and matchless gut of the dear respected Kim Jong-Un", KCNA said, in reference to the leader's courage.

The young leader, who took over after the death of his father Kim Jong-Il in December 2011, did not attend the rally.

It was the North's third test, following previous detonations in 2006 and 2009, and seismic data suggested it was significantly more powerful.

"It serves as a striking demonstration of the might of a scientific and technological power and a military power capable of manufacturing any strike," KCNA said.

North Korea said the test -- widely condemned by the international community -- was a direct response to UN sanctions imposed on Pyongyang after its long-range rocket launch in December.

Pyongyang accused the United States of leading the sanctions charge in the UN Security Council, and speakers at Thursday's rally threatened "merciless retaliatory blows" if the US pushed tougher sanctions after the nuclear test.