Myanmar's military regime has ordered the International Committee of the Red Cross out of key border areas and rejected moves to resume prison visits, the agency said on Monday.
The ICRC said in a statement that it "utterly deplores" the decision by the Myanmar authorities to close five field offices in Mandalay, Mawlamyine, Hpa-an, Taunggyi and Kyaing Tong.
"The Myanmar authorities also announced that ICRC visits to detainees would not be allowed to resume," it added.
The closure will effectively stop most of the ICRC's assistance and protection work for the most vulnerable in Myanmar, especially those held in prison or living in sensitive border areas, the aid organisation said.
"We had years of productive discussions with the Myanmar government, and our work since 1999 has had a tangible impact on people's living and security conditions," said Pierre Kraehenbuehl, the agency's director of operations.
"The ICRC is seriously worried that those most in need today will bear the brunt of the current standoff," he added.
The military junta stopped the agency making independent visits to prisons to check on the condition of detainees in December 2005, when the Red Cross rejected a request to be accompanied by government-affiliated organisations.
Myanmar's information minister Brigadier General Kyaw Hsann said earlier this month that the junta was still holding talks with the ICRC on resuming the visits.
Between 1999 and late 2005, the agency made 453 visits to prisons and labour camps across Myanmar, which has been ruled by the military since 1962.
The United Nations estimates Myanmar has more than 1,100 political prisoners, including Nobel peace laureate Aung San Suu Kyi who has spent more than a decade under house arrest.
The ICRC, which has 264 staff in the country including 24 expatriates, has met with the detained democracy leader twice since 1999.