The U.S. has called on Russia to influence militant groups occupying temporarily occupied areas on Donetsk and Luhansk regions, asking them to take the first step towards restoring peace in Ukraine, according to U.S. State Department Spokesman Mark Toner.

"The United States again calls upon Russia to use its influence with the separatists to take the first step toward peace to eastern Ukraine and ensure a visible, verifiable, and irreversible improvement in the security situation," said a statement issued by Toner on Sunday following the death of a U.S. citizen working for the mission of the Organization for Security and Cooperation in Europe's Special Monitoring Mission (OSCE SMM) in eastern Ukraine.

The statement said the death underscores the increasingly dangerous conditions under which these courageous monitors work, including access restrictions, threats, and harassment. The United States urges Russia to use its influence with the separatists to allow the OSCE to conduct a full, transparent, and timely investigation.

"This incident makes clear the need for all sides- and particularly the Russian-led separatist forces-to implement their commitments under the Minsk Agreements immediately. This includes a real and durable ceasefire, withdrawal of heavy weapons, and disengagement from the line of contact. The brave monitors of the SMM have the full support of the United States, and we condemn in the strongest terms all acts of hostility and intimidation toward SMM monitors. They must also have safe, full, and unfettered access throughout the conflict zone," the statement said.

As earlier reported, on Sunday a U.S. citizen was killed when an OSCE vehicle ran over an anti-tank landmine in Luhansk region. Two other OSCE monitors, a citizen of Czechia and Germany, were wounded in the blast and taken to a hospital for treatment.