North Korea and Iran appear to have been exchanging ballistic missile technology in violation of sanctions, a leaked UN report shows.
The report, obtained by Reuters, said regular transfers had been taking place through "a neighbouring third country", named by diplomats as China.
The sanctions were imposed on Pyongyang by the UN after it conducted a series of nuclear tests in 2006 and 2009.
They ban all trade in nuclear and missile technology with North Korea.
They also imposed an arms embargo and subjected some North Korean individuals to travel bans and assets freezes.
North Korea has twice tested nuclear devices and said in September last year that it had entered the final phase of uranium enrichment.
The country is believed to have enough plutonium to make about six bombs, but is not thought to have developed a ballistic missile capable of carrying a nuclear warhead.
'New challenges'
The report was written by a UN panel of experts monitoring Pyongyang's compliance with the sanctions.
It said that "prohibited ballistic missile-related items are suspected to have been transferred between the Democratic People's Republic of Korea [North Korea] and the Islamic Republic of Iran", using regular scheduled flights on national carriers Air Koryo and Iran Air.
For arms and related material, "whose illicit nature would become apparent on any cursory physical inspection", Pyongyang appeared to prefer the use of chartered cargo flights, Reuters quoted it as saying.
The flights would travel "from or to air cargo hubs which lack the kind of monitoring and security to which passenger terminals and flights are now subject".
This presented "new challenges to international non-proliferation efforts", said the panel.
The report said North Korea's uranium enrichment programme was "primarily for military purposes" and so Pyongyang should be "compelled to abandon" it and have it placed under international monitoring.
It also raised concerns about safety at the nuclear complex at Yongbyon, warning of an "environmental disaster" if it were to be decommissioned or dismantled without care.
The report said the transfers travelled through "a neighbouring third country". The country was not named in the report but one diplomat told the BBC some sanctions-busting takes place through China.
He said Beijing was unhappy with the experts' report, and that the Chinese member of the panel had not signed off on it.
