President Vladimir Putin on Sunday told US President Barack Obama in telephone talks that the referendum on the Ukrainian region of Crimea joining Russia was "completely in line with the norms of international law," the Kremlin said. Putin also told the US president that any Organisation for Security and Cooperation in Europe (OSCE) monitoring mission in Ukraine should cover "all Ukrainian regions" and not just Crimea. But the two leaders also agreed that "despite the differences..., it is necessary to jointly search for ways of stabilising the situation in Ukraine," the Kremlin added.