A pro-Russian official in Ukraine has called for a referendum on joining Russia to be held in the southeastern region of Zaporizhzhia on September 11. Russia claims to control more than 70 percent of the region.

The official made the comment in an interview with Russian newspaper Izvestia on Thursday. September 11 is when local elections will be held in Russia.

The official said pro-Russian forces have also discussed holding the vote on the same day as similar possible referendums in the eastern regions of Donetsk and Luhansk, as well as the southern region of Kherson.

But the official said preparations for the votes have not been progressing as expected amid continued fighting in the regions.

Analysts say Russia is seeking to annex the regions. But Ukrainian forces are mounting counterattacks to recapture occupied areas, with residents also putting up stiff resistance.

Ukrainian forces are also targeting Russian supply links. They claim to have attacked a bridge in the Kherson region, making it impassable.

Meanwhile, major US newspapers suggest Ukraine was involved in explosions that rocked a Russian air base in Crimea on Tuesday. Russia unilaterally annexed the region in 2014.

The Ukrainian government has not officially acknowledged its involvement. But President Volodymyr Zelenskyy said on Wednesday that Russia lost nine combat aircraft in the region.

Russia denied the air base was attacked and said the explosions were caused by the detonation of aircraft ammunition.