Amnesty International (AI) has accused the Sudanese government of deploying weapons to Darfur in "breathtaking defiance" of a UN arms embargo.
Amnesty says an Antonov dropped off weapons at El Geneina in July
The rights group said photographs of Russian-supplied military helicopters and a transport plane at an airport in Darfur in July showed the violations.
But Sudan's embassy in London told the BBC the pictures are suspect and accused AI of erecting a "smokescreen".
At least 200,000 people are thought to have died in the region since 2003.
A UN Security Council resolution passed in March 2005 prohibited the supply of arms to all parties in the conflict.
More than two million people have fled their homes since rebel groups rose up against the government's rule.
Khartoum and the pro-government Arab militias are accused of war crimes against the region's black African population, although the UN has stopped short of calling it genocide.
Sudan's government says the scale of the crisis has been exaggerated and only 9,000 have died in the fighting.
In a statement, AI said the photographs of Sudanese military aircraft taken by witnesses in Darfur reinforced evidence it had provided in May that Khartoum was continuing to fuel serious human rights violations there.
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
Taken at the airport in El Geneina, a town close to Sudan's border with Chad, the photographs show containers being taken from an Antonov 12 freighter plane and loaded onto military vehicles, as well as Sudanese Air Force Mi-17 and Mi-24 helicopters.
"The Sudanese government is still deploying weapons into Darfur in breathtaking defiance of the UN arms embargo and Darfur peace agreements," said Amnesty's arms control research manager, Brian Wood.
"Once again Amnesty International calls on the UN Security Council to act decisively to ensure the embargo is effectively enforced, including by the placement of UN observers at all ports of entry in Sudan and Darfur."
Local people told AI that such aircraft had been bringing in military equipment for government troops and Janjaweed militia operating in Darfur.
A Sudanese government Antonov aircraft also carried out bombing raids following an attack by the rebel Justice and Equality Movement in Adila on 2 August, Amnesty said. The nearby town of Ta'alba and two other villages in the area were also reportedly bombed.
But Khalid al-Mubarak, a diplomat with the Sudanese embassy in London, said there was "a pattern of fake photographs" and was part of an attempt to deflect public opinion from issues like Iraq, Gaza and the Democratic Republic of Congo.
Sudan bought 12 Mi-24 attack helicopters from Russia in 2005
"The Sudanese government... has sovereignty over all its territory. If it moves planes from one place to another, is this proof that there are arms?" he said on the BBC's World Today programme.
He also accused AI of becoming "part of an industry of demonising Sudan".
AI said the recent proliferation of small arms and militarised vehicles in Darfur had led to an increase in armed attacks on aid convoys and civilians.
On 31 July, the northern Rizeigat group - many dressed in Border Intelligence force uniforms - killed at least 68 members of the Tarjum, it said.
To help counter such attacks, the UN Security Council has approved a 26,000-strong peacekeeping force to expand the 7,000 African Union (AU) force currently operating in Darfur.
Officials say the United Nations African Union Mission in Darfur (Unamid) will be better equipped and will have a stronger mandate to protect civilians and aid workers.
However, the director of AI's Africa Programme said the force's mandate did not go far enough.
"If weapons continue to flow into Darfur and peacekeepers are not given the power to disarm and demobilise all armed opposition groups and Janjaweed militia, the ability of the new peacekeeping force to protect civilians will be severely impeded," Erwin van der Borght said.