North Korea is supplying banned nuclear and ballistic equipment to Iran, Syria and Myanmar using "surreptious" means to avoid international sanctions, according to a UN report released Friday.

China had blocked publication of the report which has been ready for six months, diplomats said.

North Korea is involved with "the surreptitious transfer of nuclear-related and ballistic missile-related equipment, know-how and technology" to countries including Iran, Syria and Myanmar, said the report.

A UN sanctions committee panel of experts called for heightened vigilance to stop the nuclear trade and for more detailed investigation into the sophisticated means used by North Korea to circumvent sanctions.

North Korea, known officially as the Democratic People's Republic of Korea, "employs a broad range of techniques to mask its transactions, including the use of overseas entities, shell companies, informal transfer mechanisms, cash couriers and barter arrangements," said the investigators.

Since the last sanctions were imposed in June 2009, four "non-compliance cases involving arms exports" had come to light, the report said.

It did not give details but said North Korea used "masking techniques" including mislabelling containers, falsifying ships' manifests and destination details "and use of multiple layers of intermediaries, shell companies, and financial institutions."

The North is increasingly using foreign-owned ships and modern air freight jets which can now easily get from North Korea's main airports to the Middle East without refuelling and so avoid checks.

The experts said the Security Council should consider ordering North Korea to declare all air cargos before countries give overflight clearance.

The experts "expressed concern that certain countries, such as the Syrian Arab Republic, the Islamic Republic of Iran and Myanmar, continue to be associated with the Democratic People's Republic of Korea in regard to proscribed activities and believes that special attention should be taken by all member states to inhibit such activities.

North Korea staged one nuclear test in 2006 and claims it set off another nuclear device in 2009, when the last sanctions were imposed. The UN Security Council has banned trade in nuclear and ballistic material.

The UN has named eight entities and five individuals for asset freezes and travel bans. The report said the number involved was much higher and called on countries to name other banks and other entitities that should be added to the list.

North Korea had been involved in nuclear talks with China, the United States, Russia, Japan and South Korea. But the last talks were in late 2008 and the isolated North pulled out of the negotiations the following year.

International Atomic Energy Agency director Yukiya Amano said this week that the standoff with North Korea was now "very bad".

The UN report said there were no signs that North Korea "is ready to move forward on denuclearization or to step back from its other existing weapons of mass destruction and ballistic missile development programmes."

The North "has continued to engage in activities proscribed by the relevant Security Council resolutions and has continued to boycott the six-party talks. It continues to market and export its nuclear and ballistic technology to certain other states.

China has been the North's main ally on the international stage and it had blocked the report since it was prepared in May, diplomats said.