The State Department on Thursday condemned Russian-backed separatists in eastern Ukraine for "violence and intimidation" against international monitor groups in the region.

State Department spokeswoman Heather Nauert said Thursday during a press briefing that the United States is "deeply concerned" about recent violence against civilian members of the aid group Organization for Security and Co-operation in Europe (OSCE).

"The United States is deeply concerned about an alarming pattern of violence and harassment by Russian-led separatists in eastern Ukraine directed at unarmed civilian members of the OSCE monitoring mission," Nauert told reporters.

"The latest in a series of threatening and intimidating incidents involved harassment, threats and ultimately shots being fired at retreating mission vehicles on June 20," Nauert continued. "This followed the tragic death in April of a U.S. citizen who was serving as a paramedic with monitors when his vehicle struck an explosive in separatist-controlled territory."

Nauert accused Russia of working with separatists in Ukraine to obscure what was occurring in the eastern part of the country.

"The incidents are part of a broader attempt to keep the international community from seeing what is happening in eastern Ukraine," Nauert said. "We call on Russia to use its influence to end this campaign."

Nauert also pressed Russia to allow safe access for the OSCE monitors. On June 20, a gunman attempted to break the windows and enter an OSCE vehicle monitoring ceasefire violations in Ukraine, and eventually fired shots at the retreating monitors, according to their report.

"More broadly, a lasting and durable ceasefire is urgently needed to relieve human suffering," Nauert said Thursday.

The attack occurred in the city of Yasynuvata, which is controlled by Russian-led separatists. The area has been held by separatists since 2014, when pro-Russian separatists seized several cities in eastern Ukraine, including Yasynuvata.