The ceasefire in Sudan's Darfur region has had little effect on the ground, the United States has said.
The refugees are living rough in harsh terrain
Government-backed Arab militias are reported to be continuing their attacks, a spokesman said.
The European Union may send a force to intervene in Darfur, its senior military official has said.
Some 670,000 people, mostly blacks, have fled "ethnic cleansing" by the Arab militias but remain in Sudan, with little humanitarian aid.
The 45-day ceasefire came into effect on Sunday under a deal reached between government forces and two rebel groups.
They agreed to guarantee safe passage for humanitarian aid, free prisoners of war and disarm militias.
"We do still have reports that the government-supported Arab militias are attacking parts of western and southern Darfur," State Department spokesman Richard Boucher said.
"There are also reports of continuing aerial bombardments, such as at Anka... north-west of Khartoum this [Monday] morning.
"In addition, we understand the militias remain in the vicinity of the internally displaced persons camps, occupying land that they had claimed from Africans, and effectively preventing (people) from returning to their homes," he said.
Rebels accuse the government of ignoring Darfur
However, the rebel Justice and Equality Movement (JEM) said that the ceasefire was being respected.
"The fighting has stopped entirely," Abu Bakr Hamid al-Nur told Reuters news agency.
The United Nations has warned of "ethnic cleansing" and the EU military committee chairman Gustav Hagglund told London's Financial Times newspaper that "Sudan is on the list of the UN (for some form of peacekeeping mission)."
He said the EU could send troops to Darfur, if mandated by the UN.
The UN has launched a $115m appeal for humanitarian aid for Darfur.
Following the ceasefire, a high-level UN team is due in Sudan later this month to assess the needs of those who have fled their homes.
The 10-member team led by Jan Egeland, UN undersecretary-general for humanitarian affairs, will visit each of the three regions of Darfur from 18-21 April.
Sudan analyst Eva Dadrian from the Africa Analysis newsletter says it will be hard for the government to reign in the Arab militia - the Janjaweed - which have been leading the attacks on black African civilians in Darfur.
"Darfur is a very vast region and the government will have difficulty to control these people. The Janjaweed are everywhere. They hide in the mountains," she told the BBC Network Africa programme.
The fighting in Darfur has been raging for more than a year, with rebels claiming that the Arab-dominated government is ignoring the region.
The refugees say that attacks on their villages by helicopter gunships are followed up the Janjaweed on horses and camels, who rape, kill and loot.
The Sudanese government has denied the allegations.