Russia is testing President Donald Trump with a surge of violence in eastern Ukraine and the U.S. president should give Ukraine the lethal aid it needs to defend against the attacks, Senator John McCain said in a letter to Trump on Thursday.

Renewed violence flared this week between Moscow-backed rebels and Ukraine government forces that has caused the highest casualty rate since mid-December and cut off power and water to thousands of civilians on both sides of the frontline.

“That this surge of attacks began the day after he talked with you by phone is a clear indication that Vladimir Putin is moving quickly to test you as commander in chief. America’s response will have lasting consequences,” McCain said in a letter to Trump released by his office.

Washington has supplied aid to Ukraine including drones, radar, first-aid kits, night vision and communications gear as part Democratic President Barack Obama’s strategy of providing non-lethal military assistance while focusing on sanctions and diplomacy to end the war.