Fresh satellite images suggest North Korea has completed upgrades at its main satellite launch site, fuelling speculation of a long-range rocket launch to coincide with a major political anniversary in October.

The US-Korea Institute at Johns Hopkins University said Wednesday that analysis of the new pictures showed that construction begun in spring 2015 at the Sohae launch centre had been concluded.

As a result, space launch vehicle (SLV) stages -- and payload -- can be prepared horizontally in a new launch support building, then transferred to a movable support structure and conveyed to the launch pad.

The analysis, posted on the institute's closely-watched 38North website, will reinforce predictions that the North is preparing a long-range rocket launch to mark the 70th anniversary of the founding of its ruling Workers' Party on October 10.

At a rare press conference on Wednesday, North Korea's envoy to the United Nations in Geneva, So Se Pyong, insisted that improving satellite capacity was "the right of sovereign countries".

The North insists its rocket launches are intended only to put peaceful satellites into orbit, while the US and its allies see them as banned tests of ballistic missile technology.

Asked specifically about upgrades to the main launch site at Sohae he confirmed that such upgrades had been ongoing but did not specify that they had been completed.

"This is (our) right to upgrade and upgrade and update all the technologies... for that satellite also," So said.

- Speculation over launch -

Any launch of a long-range rocket would almost certainly be viewed by the international community as a disguised ballistic missile test and result in the imposition of fresh sanctions -- significantly ramping up military tensions on the Korean peninsula.

In a similarly rare press briefing on Tuesday in New York, North Korean ambassador to the United Nations Jang Il-Hun said he could not "rule out the possibility" of an October test as part of a "grand" celebration.

Last week South Korea's Yonhap news agency cited "credible intelligence" that North Korean leader Kim Jong-Un had personally ordered a satellite launch to mark the anniversary.

According to an unnamed government source, North Korea had completed work on an extended 67-meter (220-foot) launch gantry capable of handling a rocket twice the size of the 30-meter Unha-3 rocket launched in December 2012.

The Unha-3 launch was widely condemned overseas as a ballistic missile test and triggered additional UN sanctions.

North Korea, which insisted the launch was purely scientific in nature, responded three months later by conducting its third nuclear test -- the most powerful to date.

North Korea is banned under UN Security Council resolutions from carrying out any launch using ballistic missile technology, although repeated small-range missile tests have gone unpunished.

The analysis posted on 38North sounded a note of caution, stressing that, as yet, there were "no indications" that test preparations at Sohae were underway to support a long-range SLV launch.

"There is also no public evidence to suggest that a decision has been made by the leadership in Pyongyang to move forward with a launch," it added.

Signs that a launch was imminent would include increased rail activity and the possible arrival of missile-related railcars, activity at facilities associated with rocket assembly, or the filling of oxidizer and fuel storage tanks associated with the launch pad, it said.

Visiting a newly-built satellite command centre in May, Kim Jong-Un had vowed to push ahead with further satellite launches despite the sanctions threat.

"Space development can never be abandoned, no matter who may oppose it," Kim said.

In Geneva, UN envoy So said North Korea's satellite launches had been always been unfairly misinterpreted as offensive.

"If we launch one then they say there is (a) missile," whereas if another country launches "they say it is satellite," he said. "This is the double standard."