MOSCOW (AFP) - Russian President Vladimir Putin told US President Barack Obama on Sunday, March 16, 2014, the referendum on Crimea joining Russia was fully legal, but the two leaders also agreed to work together to find ways out of the Ukraine crisis, the Kremlin said.

The telephone conversation between Mr Putin and Mr Obama, which the Kremlin said was held at the initiative of the American side, came amid soaring US-Russia tensions as initial results showed Crimea had voted to leave Ukraine and join Russia.

"(Putin) emphasised that it was fully in line with the norms of international law and the UN charter," the Kremlin said.

It added that the referendum took into account "the well-known precedent of Kosovo" which has been recognised by more than 100 countries, including the United States and all but five EU members since declaring independence from Serbia in 2008.

But the statement saide the two leaders also agreed that "despite the differences in assessment it is necessary to jointly search for ways of stabilising the situation in Ukraine".

Mr Putin also told the US President that any Organisation for Security and Cooperation in Europe (OSCE) monitoring mission should cover "all Ukrainian regions" and not just Crimea.

German Chancellor Angela Merkel told Mr Putin earlier on Sunday that she backed a bigger role in Ukraine for the OSCE, in particular in the tense east of the country.

The Russian leader, who has not recognised the new pro-West authorities in Kiev after the fall of president Viktor Yanukovych last month, told Mr Obama that Moscow was unhappy with the Ukrainian government's failure to crack down on "ultra-nationalist and radical groups".

He said such groups were "destabilising the situation and terrorising peaceful inhabitants as well as the Russian-speaking population".