Kurt Volker spoke to reporters on Thursday from Brussels, where he is meeting EU officials about keeping the sanctions in place.
Volker was appointed under President Donald Trump to push for resolution of the conflict. More than 10,000 people have been killed since 2014 in fighting between Ukrainian forces and Russia-backed separatists.
Sanctions imposed by Washington and the EU include asset freezes and travel bans.
"It's very important that we continue to keep sanctions in place given that Russia has not withdrawn its forces from Ukraine or made any steps really toward resolving the conflict there," he said.