A day after announcing a review of the original December 10-22 launch schedule, the Korean Committee of Space Technology said it was extending the window to December 29.
In a statement carried by the official Korean Central News Agency, the committee said scientists and technicians were still "pushing forward" with preparations for the mission.
"They, however, found a technical deficiency in the first-stage control engine module of the rocket... and decided to extend the launch period," it said.
North Korea says the rocket launch is a peaceful mission aimed at putting a satellite into orbit.
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.
The window extension appeared to tally with South Korean media reports, citing government analysis of fresh satellite imagery, that North Korea was replacing a faulty component in the Unha-3 rocket.
In a separate report Monday, the Chosun Ilbo -- known for its comprehensive North Korean coverage -- said a group of Iranian missile experts was in North Korea offering technical assistance for the planned launch.
The Iranians were invited after Pyongyang's last long-range rocket launch in April ended in failure, the 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," the official said.
Earlier this month, Japan's Kyodo news agency quoted a western diplomatic source as saying Iran had stationed defence personnel 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. A 2011 UN sanctions report said the two countries were suspected of sharing ballistic missile technology.
The North's decision to try and launch the rocket in winter has led analysts to suggest a political imperative behind the timing, which may have overruled technical considerations.
New leader Kim Jong-Un is believed to be extremely keen that the launch falls around the first anniversary of the death of his father and former leader Kim Jong-Il on December 17.
The possibility that the launch has been rushed has been backed by some missile experts, sceptical that the problem which caused the explosion of the same carrier in April could have been resolved in just eight months.
Despite the uncertainty surrounding the exact launch date, both South Korea and Japan said they were remaining on heightened alert.
Japan on Friday ordered its military to shoot down the rocket if it threatens the nation's territory, while Washington put anti-missile destroyers into position, ramping up pressure on Pyongyang.
UN diplomats inside and outside the Security Council have started consultations behind the scenes on what action to take if Pyongyang goes ahead with the launch.
According to Japanese reports, Japan, the United States and South Korea have agreed to demand the UN Security Council strengthen sanctions on North Korea to levels that match those on Iran.
That would include increasing the list of financial institutions, entities and individuals already subject to asset freezes.