KIEV (Reuters) - Ukraine and some of its Western allies condemned on Friday a decision by pro-Russian separatists to ban most foreign aid organizations from parts of rebel-held territory, saying the move violated the Minsk peace agreement.

On Thursday, rebel officials of the so-called Luhansk People's Republic (LNR) said they had refused accreditation to 10 out of 11 foreign humanitarian agencies, ordering them to leave LNR territory by Sept. 25.

They said the reasons for the decision were "justified" without giving further details, separatist website LITs reported.

"Ukraine expresses its deep concern, as banning the work of international organizations primarily affects civilians," presidential representative Iryna Herashchenko told Reuters.

"This is an attempt to disrupt the Minsk (peace) process."

Under the 12-point peace plan signed in Minsk, Belarus, in February, both sides committed to ensuring safe access and distribution of humanitarian aid "on the basis of an international mechanism".

The banned organizations, which include Medecins Sans Frontieres (MSF) and U.N. agencies, provide much-needed aid to civilians across eastern Ukraine, many of whose houses and livelihoods were destroyed in around 18 months of fighting between Ukrainian forces and separatists seeking independence.

The U.S. State Department condemned the decision.

"Expelling these organizations puts at risk up to three million people in separatist-held parts of Donbas by denying them access to food, water, medicine, shelter, and clothes in advance of winter," it said in a statement.

A protracted lull in fighting recently had allowed aid groups greater access to those worst-affected by the conflict, but organizations have faced increasing resistance from separatist officials, MSF Director of Operations Bart Janssens said in a statement.

"We have made every effort to be transparent and work in cooperation with the authorities ... We are therefore greatly disturbed that they have resorted to making false accusations about us in the media and tried to intimidate our team by bringing armed men into the office on multiple occasions in the last two weeks," he said.

The only international agency to be granted permission to keep working in rebel-held Luhansk was the International Committee of the Red Cross, but separately on Thursday an ICRC aid convoy of eight trucks was refused entry to separatist-controlled parts of Donetsk region.

"We would like to be able to resume and scale up our activities as soon as possible in order to be able to address humanitarian needs particularly in view of the upcoming winter," he said.

(Reporting by Alessandra Prentice and Pavel Polityuk; Additional reporting by Sandra Maler in Washington; Editing by Ralph Boulton and Alan Crosby)