A group of Iranian missile experts are in North Korea offering technical assistance with a planned long-range rocket launch condemned by the international community, a report said Monday.

The Iranians were invited after Pyongyang's last rocket launch in April ended in failure, the Chosun Ilbo newspaper said, citing a Seoul government official.

"A car seen at the... launch site has been spotted driving back and forth from the accommodation facility nearby. It is believed to be carrying Iranian experts," Chosun quoted the unnamed official as saying.

"It appears that the connection between the North and Iran in missile (development) dating back to the 1980s is more extensive than previously believed," the official said.

Earlier this month, Japan's Kyodo news agency quoted a western diplomatic source as saying Iran had stationed defence staff in North Korea since October to strengthen cooperation in missile and nuclear development.

North Korea and Iran are both subject to international sanctions over their nuclear activities and their governments share a deep hostility towards the United States.

Leaked US diplomatic cables in 2010 showed that US officials believe Iran has acquired ballistic missile parts from North Korea.

And a UN sanctions report in 2011 said the two countries were suspected of sharing ballistic missile technology.

North Korea has announced plans to launch a rocket -- ostensibly aimed at putting a satellite in orbit -- between December 10 and 22.

The United States and its allies view it as a disguised ballistic missile test banned under UN resolutions prompted by the North's nuclear tests in 2006 and 2009.

Pyongyang said Sunday it may delay the launch. It gave no reason, but analysts have suggested potential technical troubles.

Some said the new leader, Kim Jong-Un, may have been rushing the event to mark the first anniversary of the death of his father and former leader Kim Jong-Il on December 17.