The United States has spotted Irans IL-76 cargo aircraft leaving North Korea's Sunan airport on six occasions since April, the JoongAng newspaper said, quoting US and South Korean intelligence sources.
Until last year, Iranian airplanes visited North Korea about twice a year at most, the daily said.
"The Iranian cargo planes that took off from Sunan Airport flew over China and central Asian countries," an intelligence source was quoted as saying.
Although the cargo was in containers, intelligence officials have concluded that the payload was disassembled Rodong missiles, the same as were sold to Pakistan in 1998, JoongAng said.
The Rodong missile has a range of 1,300 kilometers (800 miles). Iran has been widely believed to have used North Korean technology in developing a medium-range ballistic missile test-fired in 1998.
Iran and North Korea were included last year in an "axis of evil" with Iraq by US President George W. Bush, bent on spreading weapons of mass destruction.
Meanwhile, five Japanese were arrested last week on suspicion of illegally exporting missile-linked equipment to Iran.
Tokyo has tightened controls on North Korean ships based on allegations that some have ferried missile parts to the Stalinist state and smuggled drugs into Japan.
Washington has sought to legally contain the Norths missile sales, with Bush proposing a "proliferation security initiative" to stop trade in weapons of mass destruction during his visit to Poland on May 31.
Pyongyang sent shockwaves around the world after test-firing a Taepodong long-range ballistic missile over Japan that flew into the Pacific in 1998.
According to South Korean defence ministry data, North Korea is currently testing Taepodong-1 missiles with a range of 2,500 kilometers and is also developing a longer-range Taepodong-2.
Taepodong-2 could be capable of reaching parts of the continental United States.
North Korea, locked in a nuclear stand-off with the United States since October last year, has indicated that Japan's spy satellite launch this year would free it from its commitment to a testing moratorium.
At a summit between the leaders of Japan and North Korea in September last year, North Korea pledged to extend its moratorium on ballistic missiles beyond 2003.
But North Korea has vowed to boost its military strength, insisting it would be the next target of a pre-emptive US military attack to snuff out its suspected nuclear arms ambitions.