“Putin will not talk to Obama under pressure,” said Igor Yurgens, Chairman of the Institute for Contemporary Development, a prominent Moscow think tank, and a close associate of Russian Prime Minister Dmitry Medvedev. “It does not mean forever.”

Obama and Putin last spoke over the phone on April 14, a call that the White House said was initiated at Moscow’s request. Obama urged Putin in the call to end Kremlin support for armed, pro-Russian activists creating unrest in eastern Ukraine. Obama also warned that the U.S. would impose more “costs” on Russia if Putin continued his current course. According to the Kremlin’s readout of the call, Putin denied Russian interference in eastern Ukraine and said “that such speculations are based on inaccurate information.”

Obama and Putin have spoken to each other about Ukraine regularly over the past weeks, including calls on March 28, March 16, and March 6. But that these calls are now on hold for the indefinite future, due to their lack of progress and frustration on both sides.