The intended destination of a plane carrying 35 tonnes of arms from North Korea and impounded in Thailand was tonightstill unclear, with none of the governments apparently linked to the seized flight admitting any responsibility for its cargo.

Ukraine today said it had launched an investigation into the Ilyushin-76 aircraft, amid speculation that it may have been transporting arms to Iran as part of a North Korean smuggling network used to fund North Korea's banned nuclear weapons programme.

Ukrainian sources indicated the plane had originally set off from Belarus. Belarus's foreign ministry denied the report, but confirmed that one of its citizens – Mikhail Petukov – had been on board, working as a flight engineer.

According to Ukrainian officials, the plane travelled via Ukrainian airspace and refuelled at an airport near Kiev. It set off again on 8 December without a cargo to North Korea. The plane picked up a shipment of portable grenade launchers, an anti-aircraft missile system and other weapons from Pyongyang, North Korea's capital.

Thai officers seized the aircraft on Saturday at Bangkok's Don Muang airport, acting on tip-offs from US and other intelligence agencies that the plane had been carrying North Korean weapons in contravention of a UN security council ban on arms exports.

Today Bangkok's criminal court extended the detention of the plane's five-man crew, four of whom come from Kazakhstan. Kazakhstan tonight denied any knowledge of the illegal arms shipment. It said the plane was registered in Georgia on 7 October and had been leased to a New Zealand company.

The crew has been charged with possession of heavy weapons and misstating the nature of the cargo, officially described as "oil-drilling equipment". Crew members claim they had no idea they were carrying weapons.

"They thought it was a civilian freight flight carrying oil drilling pipes and other equipment for oil drilling," defence lawyer Somsak Saithong said, according to Reuters. Saithong said the crew had delivered such equipment "a few times" in the past, adding that three of the crew were trained pilots. According to military sources, the cargo also included missile tubes, spare parts and other heavy weaponry. Experts are now examining the contents at a secluded military airport.

Today, government spokesman Panitan Wattanayagorn said the aircraft was supposed to be flying to the Sri Lankan capital, Colombo. But he said the authorities were investigating whether the flight plan was misleading, and the final destination was in the Middle East, noting the aircraft had recently stopped in the United Arab Emirates.

"We believe after Colombo there may have been another destination," he said, adding that, according to the crew, the plane had planned to refuel in the United Arab Emirates and Azerbaijan before flying to its "final destination" in the Ukraine.

"We are taking all of this with a pinch of salt. We will have to verify all the claims, including whether the passports are real," Panitan said, adding police still had little information about who the crew members were, where they have been trained and whether they were linked to a terrorist organisation.

North Korea was hit in June with fresh UN sanctions to punish it for a nuclear test in May. These are aimed at cutting off its arms sales, which earn the isolated and impoverished state more than $1bn a year. The North's biggest arms sales come from ballistic missiles, with Iran and other Middle Eastern states major customers, US government officials suggest.