A convoy of vehicles from the Organisation for Security and Co-operation in Europe (OSCE) -- led by a police car and followed by two buses carrying the observers and a large number of cars waving Ukrainian flags -- were stopped at a checkpoint manned by armed men as they tried to enter Crimea for a second day.
The observer mission is a crucial part of the so-called "off-ramp" US President Barack Obama is pushing to de-escalate a crisis in Ukraine that threatens to splinter the ex-Soviet nation of 46 million along its cultural divides.
"Should the people of Crimea decide to join Russia in a referendum, we... Will unquestionably back this choice," said speaker of the upper house Valentina Matviyenko.
"We will respect the historic choice of the people of Crimea," said her lower house counterpart Sergei Naryshkin.
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Moscow police said more than 65,000 people attended a rally outside the Kremlin supporting Russia's full annexation of the region of two million people.
It marked the leaders' second lengthy phone call in five days and both sides described it as tough.
The White House said Obama "emphasised that Russia's actions are in violation of Ukraine's sovereignty and territorial integrity, which has led us to take several steps in response, in coordination with our European partners."
The European Union earlier firmed its resolve to impose stiff sanctions on Russia while also vowing to sign an historic trade pact aimed at pulling Kiev out of Moscow's orbit before Ukraine holds snap presidential polls on May 25.