The US state department has condemned an incident in eastern Ukraine where pro-Russian rebels held civilian monitors at gunpoint.
The OSCE said on Friday that the rebels had surrounded one of its teams near Donetsk, opening fire and seizing a drone they were trying to launch.
The US statement called on Russia and the separatists to observe a fragile six-day-old ceasefire.
Ukrainian forces and rebels have been fighting since 2014.
The ceasefire was agreed after an upsurge in violence last month. More than 9,800 people have died since the conflict began.
The Organization for Security and Co-operation in Europe has an observer mission in the conflict area to monitor the truce.
"We condemn Friday's targeting of OSCE Special Monitoring Mission (SMM) monitors and the seizure of a SMM unmanned aerial vehicle by combined-Russian separatist forces," the state department said.
"We call on Russia and the separatist forces it backs to immediately observe the ceasefire, withdraw all heavy weapons, and allow full and unfettered access to the OSCE monitors."
The statement also urged the rebels to halt attacks on civilian infrastructure.
The OSCE monitors were in the town of Yasynuvata trying to gather information about the apparent recent shelling of Donetsk water filtration plant when they were surrounded by rebels.
The plant, which is close to the front line, had announced on Friday that it was closing because of the attacks.
The rebels pointed guns at the monitors, the OSCE said, and one fired shots that landed near them.
Mission chief Ertugrul Apakan said on Friday: "Firing at unarmed civilian monitors is not only a direct threat to the lives of brave men and women doing their best to bring peace to Ukraine.
"It is a direct challenge to the collective will of the 57 OSCE participating states, and to the Minsk agreements."
A peace deal for eastern Ukraine was signed in the Belarusian capital of Minsk in February 2015 but since then previously agreed ceasefires have not held for long, with both Ukrainian government forces and the rebels repeatedly accused of breaking them.