(CNN) -- Medecins Sans Frontieres slammed the United Nations Thursday for what it said was an "unacceptable" humanitarian response to the crisis in the Central African Republic.
In an open letter, the group, also known as Doctors Without Borders, said it had "deep concern" over the United Nations' humanitarian agencies' "appalling performance" in the country over the past year.
This failure, it said, has left tens of thousands of vulnerable people without the help they need.
Violence has plagued the Central African Republic since a coalition of rebels deposed President Francois Bozize in March, the latest in a series of coups since the nation gained independence.
The past week has seen a surge in fighting in the capital, Bangui, and in the town of Bossangoa, 185 miles north, where many of those displaced by months of conflict between Muslim and Christian militias have sought refuge in camps.
French troops have now been deployed alongside African forces in a peacekeeping mission aimed at restoring security, protecting civilians and ensuring access to humanitarian aid.
The MSF letter, addressed to Valerie Amos, the U.N. Under-Secretary-General for Humanitarian Affairs and Emergency Relief, is scathing in its assessment of the U.N. response on the ground.
As the situation deteriorated in the past three months, "there has been no evidence of an adequate humanitarian reaction to the needs generated by repeated outbreaks of violence," the letter said.
"The only actions undertaken by UN aid officials have been the collection of data related to the fighting and a few assessments confirming the need for an immediate response. Repeated evaluations in the face of glaring needs, and numerous coordination meetings, have not led to any concrete action around the main hotspots."
The letter gives two examples in support of its criticism.
"MSF has repeatedly asked UN agencies to deliver food, tents and soap to the more than 15,000 people displaced in the vicinity of Bangui's airport, without any reaction," it said.
In the second example, U.N. officials in Bossangoa were on security lockdown inside a compound run by African-led peacekeeping forces, the letter said. They "did not even provide assistance to the displaced sheltering inside the same compound, forcing MSF to intervene once more."
The same officials remained on lockdown for days after the fighting, it added, "abandoning the more than 30,000 displaced persons in the main Bossangoa camps."
MSF staff instead moved through the camps to give help, the letter said. That assistance included taking care of the wounded, digging latrines and providing access to drinking water.
The U.N. said Thursday that U.N. agency personel have deployed to the field as security conditions permitted, adding that they are "working closely" with MSF and other key NGOs who have maintained a strong presence in CAR during the crisis.
"As this crisis worsens, it is important that all efforts and resources are focused on delivering aid to people in desperate need. As in all crises, there will be a time for evaluation of the humanitarian response, but now is the time for action," the U.N. said.
Medecins Sans Frontieres, which has been present in the Central African Republic since 1997, says it is currently running seven regular projects and four emergency projects there. It has more than 100 foreign personnel and about 1,100 local staff in its teams, providing free medical care to nearly 400,000 people.
The group believes the United Nations has overestimated the security risks in the Central African Republic, thereby restricting its agencies' ability to help "people in an extreme state of need."
An MSF news release Thursday, published alongside the letter, said U.N. humanitarian organizations had now decided to ramp up their response in the Central African Republic.
But while this is welcome, the group said, it is long overdue.
"This late decision must have a real impact and initiate radical and immediate change to the way UN humanitarian agencies respond to the crisis," said Bart Janssens, MSF director of operations.
More than 415,000 people -- nearly 10% of the population -- have been internally displaced by the fighting, according to the United Nations, and 68,000 more have fled to neighboring countries.
CNN's Nana Karikari-apau contributed to this report.