The United States on Thursday accused pro-Russian rebels in eastern Ukraine of a campaign of "violence and harassment" against unarmed OSCE teams monitoring the conflict.
State Department spokeswoman Heather Nauert said Washington is "deeply concerned" about the situation, describing the rebels as "Russian-led, Russian-funded and Russian-trained."
A ceasefire between Ukrainian troops and the separatists is to come into effect this weekend, under the watchful gaze of the Organization for Security and Co-operation in Europe.
But previous truces in the two-year-old conflict have broken down, and Washington accuses Moscow of intimidating OSCE monitors to cover up what is going on on the ground.
Nauert said that in a June 20 incident, separatist forces had fired at retreating OSCE vehicles, and noted that an American paramedic had been killed in an explosion in April.
"The incidents are part of a broader effort to keep the international community from seeing what is happening in eastern Ukraine," she alleged.
"We call on Russia to use its influence to end this campaign of intimidation and honor its commitment to allow free, full and safe access to the OSCE monitors."