China and Russia have denied claims by Amnesty International that they are supplying arms to Sudan for use in Darfur, in breach of a UN arms embargo.
Amnesty says a white Antonov 26 was spotted at Nyala in March
A report by the rights group says the weapons end up in the hands of the government-backed Janjaweed militia.
It also includes apparent photographic evidence of Sudan using military aircraft camouflaged white in Darfur.
China said its exports to Sudan were legal, limited and on a small scale. Russia also denied any embargo breach.
A Russian foreign ministry official said Moscow "unswervingly" observes the UN restrictions.
Under the terms of a UN Security Council resolution passed in March 2005, an embargo is in place on the supply of arms to all parties in the conflict in Darfur.
The Sudanese ambassador to the UN, Abdel Mahmood Abdel Haleem, said the Amnesty allegations were "baseless and unfounded".
More than 200,000 people have died during a four-year conflict in Darfur, and the Janjaweed militia are accused of displacing and killing tens of thousands of people.
In its report, Amnesty calls on the UN Security Council to strengthen the arms embargo on Darfur, which was extended in March 2005 to cover all parties.
Amnesty says it is "dismayed" that two permanent members of the UN Security Council are "allowing ongoing flows of arms to parties in Sudan".
UN ARMS EMBARGO
Imposed by Resolution 1591, 29 March 2005
Cuts the supply of arms to all parties to the conflict in Darfur
Some nations regard arms exports to Sudan's government as allowed under the embargo
Sudan is permitted to run humanitarian flights into Darfur, but only with UN permission - never requested
The report provides photographs of what it says were a Russian Mi-24 attack helicopter at Nyala in Darfur and Chinese Fanfan fighters at Nyala earlier this year.
It also pictures an all-white, Russian-built Antonov 26 military plane, with the registration code ST-ZZZ.
It says it appears there are "three planes with this registration number" and said it was "highly likely" they were used in bombing raids.
The Amnesty report backs up a UN study, leaked to the New York Times last month, which said Sudan was painting aircraft white to make them look like UN planes - a practice banned by the Geneva Convention.
"The use of all-white aircraft and helicopters... in Darfur is in violation of applicable norms of international humanitarian law," the new report says.
Sudan denies using any white aircraft for military purposes, but says it has some white helicopters to transfer officials.
Amnesty says its report is based on eyewitness accounts from Darfur and "confidential sources".
The Amnesty report backs the UN findings suggesting that Sudan is flying weapons into Darfur in breach of UN Security Council resolutions, a claim denied by Sudan.
But Amnesty says Sudan is "routinely failing to seek [UN] approval to move weapons... into Darfur", it says, and is importing weapons it knows will be used to target civilians.
Chinese-made Fantan fighters have been seen at Nyala, Darfur
The human rights group says Russia and China are aware of the eventual uses of arms exported to Sudan.
It cites 2005 trade figures as showing China sold $24m and Russia $21m of military material to Sudan.
Amnesty has also accused Kuwait, Saudi Arabia and Belarus of supplying arms.
China has a close relationship with Sudan, increasing its military co-operation with Khartoum earlier this year.
The relationship is based around Khartoum's plentiful supply of oil, which China needs to fuel its booming economy, says the BBC's Daniel Griffiths in Beijing.
However, Amnesty now says it wants a list made of all items prohibited for transfer and for UN personnel to be stationed at all ports of entry in Sudan.
Amnesty also wants all UN states to suspend the transfer of any arms and ammunitions likely to be used by the parties in Darfur.
But Mr Haleem said military assets were simply being moved around the country.
Mr Haleem told the BBC: "We are moving these military assets to their respective places. We are not using these aircraft for any military function in Darfur."