MSF says the dangerous environment in Somalia has curtailed its work
Aid agencies are struggling to help those most in need as operating environments become increasingly hazardous, a medical charity said.
In a list of "top 10" humanitarian crises, Medecins Sans Frontieres said conditions in Somalia, DR Congo and Sudan were particularly challenging.
Threats in Somalia and Pakistan had forced MSF to limit its work, it said.
Fighting in Sudan's Darfur region and in DR Congo had also left it unable to reach vulnerable populations, it said.
Nicolas de Torrente, director of MSF-USA, told the BBC that the list, published annually, aimed to focus attention on places where people were suffering most.
MSF's TOP 10 CRISES
Somalia; Violence is worsening and healthcare has collapsed
DR Congo and Pakistan; Fighting in eastern DRC and north-west Pakistan has left many displaced
Sudan; Darfur violence continues; aid is badly needed in Southern Sudan
Ethiopia's Somali region: Fighting has isolated population from services and aid
Burma and Zimbabwe; Government neglect and economic chaos are worsening health crises
Iraq; Aid is needed but agencies have been targeted for attack
Global; Malnutrition kills millions of children each year, while tuberculosis is increasingly threatening those with HIV/Aids
"The issue for us is how to reach these people and how to try to provide them with meaningful assistance and there we find that there are many obstacles," he said.
"Governments don't want us to be present - they fear the exposure that comes with that. They want to deny assistance to these populations and, increasingly, we see attacks, directed attacks, against aid workers."
In Somalia, MSF said, attacks and threats against aid workers had forced it to curtail its operations, significantly reducing what it could offer an already weakened population.
The same thing had occurred in north-west Pakistan, MSF said, rending it less able to assist those fleeing the fighting there.
In eastern DR Congo, fighting had left many areas too dangerous for aid workers, even though people were in desperate need of help - a situation that was repeated in the Sudanese region of Darfur.
The charity also flagged up health crises in both Burma and Zimbabwe.
In Burma, it said, hundreds of thousands of people were dying from Aids because the government was failing to act.
In Zimbabwe, soaring inflation had left many HIV/Aids patients without even the bus fare needed to visit a clinic, it added.
Other crises included in the MSF list were childhood malnutrition around the world and an aid deficit in Iraq.