(Adds quotes from briefing)
By Susan Heavey and Steve Holland
WASHINGTON, March 18 (Reuters) - The United States condemned Russian President Vladimir Putin's move on Tuesday to formally annex Ukraine's Crimea region and vowed to impose more sanctions, the White House said.
"This action - the results of the referendum and the attempt to annex a region of Ukraine - will never be recognized by the United States and the international community," White House spokesman Jay Carney said.
The Obama administration is preparing to expand the list of those facing asset freezes and travel bans under the sanctions order imposed on Monday, Carney also told reporters in a briefing.
"More is coming," Carney said. "Work is being done to make further designations" under the sanctions.
"If Russia refuses to change course, it will incur more costs, imposed by us, imposed by our friends and allies around the world and imposed in general by the global economy," he said.
Earlier on Tuesday, Putin signed a treaty making Crimea part of Russia following a disputed referendum in the Ukrainian region on Sunday and in defiance of sanctions by the United States and the European Union.
Carney did not say when the additional sanctions would be announced or give any other details.
"Costs will increase until Russia changes course," he added. (Additional reporting by Doina Chiacu; Editing by Bill Trott; Editing by Sandra Maler)