WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson said on Friday he had chosen an envoy to lead U.S. diplomacy on the Ukraine crisis, adding that he had moved to fill the position at the urging of Russia’s leader.

Former U.S. Ambassador to NATO Kurt Volker, a longtime diplomat, will be responsible for advancing U.S. interests as set out in the 2015 Minsk agreement to curb the conflict in Ukraine’s Donbass region. He will accompany Tillerson to Kiev on Sunday, the State Department said.

Russia annexed Ukraine’s Crimea region in 2014, and Kiev accuses it of backing pro-Russian separatists in eastern Ukraine, an allegation the Kremlin denies. The Minsk agreement called for a ceasefire in eastern Ukraine, the withdrawal of heavy weapons from the front line and constitutional reform to give eastern Ukraine more autonomy.

“At the request of President (Vladimir) Putin, the United States has appointed ... a special representative for Ukraine, Ambassador Kurt Volker,” Tillerson told reporters after a meeting between Putin and U.S. President Donald Trump on the sidelines of the G20 summit in Hamburg, Germany.

Ukrainian President Petro Poroshenko welcomed the choice, writing in a message on Twitter that it would help end what he called Russian aggression and restore Crimea to Ukraine.

“Important & timely move in the interests of ending Russian aggression and restoration of Ukraine’s territorial integrity, including Crimea,” Poroshenko tweeted.

Volker was a career diplomat who served as permanent representative to the North Atlantic Treaty Organization under Republican President George W. Bush and Democratic President Barack Obama. He is currently a foreign policy and national security expert at the McCain Institute for International Leadership at Arizona State University.

“Kurt’s wealth of experience makes him uniquely qualified to move this conflict in the direction of peace,” Tillerson said in a statement. “The United States remains fully committed to the objectives of the Minsk agreements, and I have complete confidence in Kurt to continue our efforts to achieve peace in Ukraine.”

Julie Smith, a former Pentagon official who worked on European and NATO policy during the Obama administration, praised the choice of Volker as Ukraine envoy, but said she was puzzled at Tillerson’s statement that he filled the position at Putin’s request.

“So Ukraine didn’t matter enough to this administration to have them appoint a special envoy in the first place?” said Smith, now a senior fellow at the Center for a New American Security. “It was a bizarre word choice.”