U.S. Secretary of State Rex Tillerson | Pool photo by Chris Kleponis/Getty Images Tillerson asks Russia to ‘lower the level of violence’ in Ukraine Officially, the Russian government denies its military is operating in eastern Ukraine.

WASHINGTON — Secretary of State Rex Tillerson on Tuesday asked his Russian counterpart to reduce the levels of violence in eastern Ukraine, where Russian troops have intervened on the side of separatist militias in their ongoing clash with the Ukrainian government.

“Underscoring the U.S. concern with the rising violence in eastern Ukraine, the secretary requested that Russia return its representatives to the Joint Center on Coordination and Control and lower the level of violence,” State Department spokeswoman Heather Nauert said in a statement emailed to reporters Wednesday.

Officially, the Russian government has denied that its military is operating in eastern Ukraine, despite photographic and other evidence to the contrary. Eastern Ukraine has been the site of clashes between Ukrainian government forces and pro-Russia militias since 2014, when Ukraine’s Moscow-aligned president was ousted by protesters in Kiev.

In response, Russia quickly annexed Crimea away from Ukraine, a step that most nations have refused to recognize as legitimate. The Kremlin has also continued to aid pro-Russia fighters in the eastern part of Ukraine, where ties to Russia are stronger.

Nauert’s statement also said Tillerson and Russian Foreign Minister Sergey Lavrov agreed to continue to seek a diplomatic solution to the threat posed by North Korea’s nuclear weapons program and to support negotiations toward a peaceful end to the ongoing civil war in Syria, where the U.S. and Russia have supported opposing sides.