KIEV (REUTERS) - Ukraine's Crimea has closed its airspace to commercial flights, five days ahead of a referendum organised by its pro-Russia authorities called to join the Black Sea region to Russia.

A Ukrainian airline plane was turned back on Tuesday on its way from Kiev to Simferopol, the region's main city, and had to return to the Ukrainian capital.

The captain told passengers that the Crimean authorities had closed airspace to all commercial flights and there had been no flights on Monday either.

Tension has increased in the region ahead of Sunday's referendum which pro-Russian leaders, backed by Russian forces, have called despite it being denounced as illegal by Ukraine's new rulers and Western governments.