Secretary of State Rex Tillerson's team issued a pointed reminder that Russia bears responsibility for the deaths of 298 civilians killed when an airliner was downed over Ukraine in 2014.

Malaysia Airlines Flight MH17 was shot down on July 17, 2014, as Ukrainian forces clashed with a separatist movement supported by Russian special forces in the eastern part of the country. Russian President Vladimir Putin has denied responsibility for the incident, but Dutch investigators concluded that the plane was downed by a surface-to-air missile launched from separatist territory.

"Russian-led forces in eastern Ukraine fired the surface-to-air BUK missile — brought into sovereign Ukrainian territory from Russia — that took down flight MH17," State Department spokeswoman Heather Nauert said Monday.

That rebuke, delivered while marking the third anniversary of the tragedy, comes as the Trump administration is trying to jump-start peace talks in Ukraine. Tillerson appointed a special envoy known for his hawkish foreign policy views to represent the U.S. in negotiations with Moscow, as a pair of ceasefires brokered by Western powers during former President Barack Obama tenure have failed to hold. The fighting in eastern Ukraine expanded Russian influence in the country, which had grown due to the annexation of Crimea.

MH17 was downed a time when Russian leaders were still denying that their military was fighting alongside separatists in Ukraine. Boris Nemtsov, a Putin critic who was assassinated in 2015, alleged in a report he was writing when he died that Russian special forces went to eastern Ukraine in the summer to blunt a Ukrainian offensive.

"Starting in June 2014, the armed forces of Ukraine undertook a successful offensive against the separatists' positions," Nemtsov's report says. "However, on August 19-20, there was a turning point on the front, and the Ukrainian offensive broke down. This became possible thanks to a massive reinforcement that arrived from the territory of the Russian Federation, including military equipment and regular army units."

Russian Foreign Minister Sergey Lavrov acknowledged military involvement in Ukraine during a recent foreign policy forum alongside Henry Kissinger. "Had we not done what we did, we would have betrayed our civilisation which our forefathers developed over centuries and who then spread it over vast territories," Lavrov said.

Nauert emphasized U.S. support for a Dutch-led effort to prosecute anyone involved in the downing of the plane, which was flying from Amsterdam to Kuala Lampur. "The United States will continue to support the efforts of the Joint Investigation Team and urges other states to cooperate fully in order to ensure those responsible are brought to justice," she said. "We have full confidence in the ability of the Dutch criminal justice system to conduct a prosecution that is comprehensive, objective and just."