Doctors Without Borders teams were threatened and stopped from reaching areas in Myanmar hit by communal bloodshed, the charity said yesterday.
More than 100,000 people have been displaced and dozens killed since June in Rakhine state in two eruptions of violence between Rakhine Buddhists and Muslims, mainly from the Rohingya minority.
Neighbourhoods have been torched since the second wave of unrest began last month, prompting another exodus and leaving already overcrowded camps struggling to cope with a growing humanitarian crisis.
Doctors Without Borders said its teams faced "ongoing antagonism generated by deep ethnic divisions", which had stopped them from treating both those newly displaced and patients of its longer-term projects in the region.
"That we are prevented from acting and threatened for wanting to deliver medical aid to those in need is shocking and leaves tens of thousands without the medical care they urgently need," said Joe Belliveau, the charity's operations manager.
Some ethnic Rakhine leaders have campaigned against international aid agencies in recent months, claiming they favour the Rohingya.
Aid groups deny the accusations.
"The animosity is rooted in a small minority of the population but a very vocal one. They must accept that a very basic medical act is not somehow supporting the other side," Mr Belliveau said, adding that the agency did not "play favourites".
He said threats in letters, pamphlets and on Facebook used "highly vitriolic" language that caused staff to fear for their safety.
"We are only out there to provide people with health care who need it the most ... it is outrageous that this should be cut off. Anybody who needs health care should be getting health care," he said.
Some of those who fled their homes were "very exposed" and medical workers had found people with a variety of injuries, including those who had been burnt, stabbed or wounded by arrows or bullets, Mr Belliveau said.
Myanmar's 800,000 Rohingya, described by the UN as among the world's most persecuted minorities, are seen by the government and many in the country as illegal immigrants from neighbouring Bangladesh.
They make up the vast majority of those displaced in the fighting.
Egypt summoned Myanmar's ambassador in Cairo yesterday over violence against the Rohingya.
The country's foreign ministry called on Myanmar to take steps to immediately end the violence and bring its perpetrators to justice, a spokesman said.
Egypt previously summoned the ambassador in August to protest against the ethnic clashes that began in June.
* Agence France-Presse