U.S. President Donald Trump’s special envoy Keith Kellogg said in Kyiv on Monday that officials are “working very, very hard” on efforts to end the three-year war between Russia and Ukraine, as a lack of progress fuels doubts about whether a peace settlement could be on the horizon.

U.S. President Donald Trump renewed a threat to impose sanctions on Russia on Friday if there is no progress toward a peaceful settlement in Ukraine in two weeks, showing frustration at Moscow a week after his meeting with Russian President Vladimir Putin in Alaska.