President Obama in a telephone call with Russian President Vladimir Putin told the leader that western nations were “prepared to impose additional costs on Russia” after Putin endorsed a referendum Sunday in which Crimea chose to secede from Ukraine.

Obama told Putin that the Crimea vote -- more than 95 percent favored joining Russia -- “violates the Ukrainian constitution and occurred under duress of Russian military intervention,” according to a White House readout of the call.

“President Obama reiterated that a diplomatic resolution cannot be achieved while Russian military forces continue their incursions into Ukrainian territory and that the large-scale Russian military exercises on Ukraine’s borders only exacerbate the tension,” the White House added.

In their description of the call between the two leaders, the Kremlin said that Putin told Obama the Crimean referendum was a legal vote — and that the U.S. should do more to combat “radical” elements in Ukraine.

U.S. officials have expressed fears that Putin would shift his forces into southeastern Ukraine. Putin has dismissed such thinking, but Russian officials have also downplayed the extent to which troops were deployed to Crimea.

The Obama administration, in partner with its European allies, is expected to impose sanctions on leaders in the Putin regime as early as Monday.

Yet, the president is still trying to achieve a last-second solution to the Ukrainian crisis — one that looks increasingly unlikely given the chronic impasse.

Obama “noted that the Ukrainian government continues to take concrete steps that would allow for the deescalation of the crisis, particularly as it prepares for elections this spring and undertakes constitutional reform,” the White House said, “and he asked that Russia support the immediate deployment of international monitors to help prevent acts of violence by any groups."