LONDON (Reuters) -The Russian-backed self-proclaimed Donetsk People's Republic in eastern Ukraine may consider joining Russia once it controls all of Ukraine's Donetsk region, its news outlet cited separatist leader Denis Pushilin as saying on Tuesday.

       "As for joining the Russian Federation, as for the wish and aspiration, they have been clearly traced since 2014 - the desire to be in Russia," Pushilin was quoted by the Donetsk News Agency as saying.

       "But now the main task is to reach the constitutional borders of the republic. Then we will determine that," he said.

       The comments come two days after the leader of the other Russian-backed eastern Ukrainian rebel region Luhansk said it may hold a referendum on joining Russia.

       Kyiv said any such vote would have no legal basis and would trigger a stronger international response.

       Three days before ordering the Feb. 24 invasion of Ukraine, President Vladimir Putin recognised the breakaway territories in Luhansk and Donetsk as independent states, though the rest of the world considers them part of Ukraine.

       Ukraine, which says it is fighting for its existence against what it casts as an imperial-style land grab by Russia, has repeatedly said it will never agree to Russia's annexation of its territory - the hardest part of peace talks with Moscow.

       (Writing by Conor Humphries; Editing by Mark Trevelyan)