The United Nations says the Sudanese government has carried out fresh bombing raids on rebels in the province of Darfur using helicopter gunships.
Darfur has been described as the world's worst humanitarian crisis
Refugees are also coming under attack in southern Darfur from the pro-government Janjaweed militia, it said.
The lobby group Human Rights Watch has also accused Sudan of breaking pledges to rein in the militias.
In a report, HRW said that the outside world had failed to prevent atrocities that have forced many people to flee.
The Sudanese ambassador to Britain, Dr Hassan Abdin, rejected Human Rights Watch accusations that Janjaweed militiamen were being incorporated into government security forces in Darfur.
"To talk of governments acting in connivance and collaboration with the Janjaweed is not true," Mr Abdin told the BBC's World Today programme.
He said Sudan was acting to rein in the militia and had arrested and tried 100 of its members.
The UN's office for the co-ordination of humanitarian affairs is highly critical of the Sudanese government.
It says that despite promising to improve humanitarian access, Khartoum has been placing so many restrictions on aid workers that access has actually got worse over the past week.
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
Flights operated by the World Food Programme have been inexplicably grounded, for example, while other aid agencies say they are having difficulty recruiting local staff because of government-imposed restrictions and delays.
The UN refugee agency says that Sudan's government is putting pressure on internally displaced people to return to their villages, despite concerns over safety.
Last week, the Sudanese government and the UN agreed on a plan to tackle the Darfur crisis, which included measures to provide safe areas around certain towns and villages.
"We have interviewed people in hospital who tell us they
have gone back to the villages... and have been shot by Janjaweed raiders," says United Nations High Commissioner for Refugees spokesman Peter
Kessler in Geneva.
The UN's new director of operations for Darfur, Jean-Marie Farcury, is beginning a visit to Sudan's neighbour Chad on Wednesday amid concern for the future of the estimated 200,000 refugees there.
But HRW says the international community is failing in its duties.
"Despite growing global attention to the crisis in Darfur, neither the international community nor the Sudanese government has taken the steps needed to ensure protection for civilians on the ground," said the group's Peter Takirambudde.
"Rape, assaults and looting continue daily even as more people are being driven from their homes."
Meanwhile, the US state department says it is investigating a claim by Amnesty International that the Sudanese authorities have harassed and detained refugees in Darfur who talked to Secretary of State Colin Powell and other foreign officials.
Amnesty says there have been multiple arrests in Darfur in recent weeks of people who have expressed their views on the continuing violence to foreign journalists and to foreign officials.
Meanwhile, it is feared a giant plague of locusts sweeping across Africa could reach refugee camps in Chad in a week's time, exacerbating food shortages and operational difficulties, and adding to the suffering of the people there.