Image caption The most recent launch came from the Sohae site, but North Korea is also upgrading Musudan-ri

North Korea appears to be upgrading one of its two rocket launch sites, US experts say, perhaps in a move to test bigger rockets.

"Important progress" had been made at Tonghae Satellite Launching Ground since October 2012, the analysis from the 38 North website said.

Activities around the new launch pad also revealed possible evidence of assistance from Iran, it said.

Pyongyang used a three-stage rocket to put a satellite into space last year.

That launch - condemned by the UN as a banned test of missile technology - took place at the Sohae launch site.

But previous unsuccessful attempts in 2006 and 2009 took place at the Tonghae site, which is also known as Musudan-ri.

The analysis from 38 North, the website of the US-Korea Institute at Johns Hopkins School of Advanced International Studies in Baltimore, was based on satellite imagery.

Construction of the new launch pad was continuing, it said, with images showing Pyongyang would be able to test rockets "perhaps three to four times the size of the Unha [launched in December 2012] when construction is completed, possibly in 2016".

Two new design features were similar to those used at the Semnan Launch Complex in Iran, it said.

The images also confirmed activity at the old launch pad.

"That activity may be related to another round of modifications intended to support future launches of the Unha rocket or possibly another liquid-fuelled missile," 38 North said, while cautioning that more information was needed.

North Korea last week conducted its third nuclear test, claiming to have successfully detonated a smaller but more powerful device than in previous tests.

The move drew immediate condemnation from the UN Security Council.

Observers fear North Korea is working towards creating a nuclear device small enough to fit on a long-range missile.