Russia views efforts to end its three-year war with Ukraine as “a drawn-out process,” a Kremlin spokesman said Monday, after U.S. President Donald Trump expressed frustration with the two countries’ leaders as he tries to bring about a truce.

“We are working to implement some ideas in connection with the Ukrainian settlement. This work is ongoing,” Dmitry Peskov said in a conference call with reporters.

“There is nothing concrete yet that we could and should announce. This is a drawn-out process because of the difficulty of its substance,” the Kremlin spokesman said when asked about Trump’s anger at Russian President Vladimir Putin’s comments dismissing Ukrainian President Volodymyr Zelenskyy’s legitimacy to negotiate a deal.

Russia has rejected a U.S. proposal for a full 30-day halt in the fighting. The feasibility of a partial ceasefire on the Black Sea, used by both countries to transport shipments of grain and other cargo, was cast into doubt after Kremlin negotiators imposed far-reaching conditions.

Trump promised during last year’s campaign that he would bring Europe’s biggest conflict since World War II to a swift conclusion. Peskov didn’t address Trump’s criticism of Putin on Sunday, when he said he was “angry,” that Putin had questioned Zelenskyy’s credibility as leader.

But the spokesman said that Putin “remains absolutely open to contacts” with the U.S. president and was ready to speak to him.