The United States is reviewing its military presence in Europe as a result of Russia's intervention in Ukraine, the White House said Tuesday as President Barack Obama began a four-day visit to the continent. Obama will request from Congress a "European Reassurance Initiative" of up to $1 billion to increase U.S. military rotations in Europe, the White House said in a statement. The effort would involve increased participation by the U.S. Navy in North Atlantic Treaty Organization (NATO) naval force deployments, including "more persistent deployments" to the Black and Baltic Seas, it said. "We are reviewing our force presence in Europe in light of the new security challenges on the continent," the White House wrote. "These efforts will not come at the expense of other defense priorities, such as our commitment to the Asia-Pacific rebalance."