Firefighters putting out a large-scale fire at a food warehouse in Severodonetsk, Lugansk region, destroyed following Russian shelling. AFP
"I think that in the near future a referendum will be held on the territory of the republic, during which the people will... express their opinion on joining the Russian Federation," Russian news agencies quoted Leonid Pasechnik as saying.
"For some reason, I am sure this will be the case," he said.
Russia launched its military action in Ukraine in late February, saying it was acting in defence of the self-proclaimed Donetsk and Lugansk republics in the country's east.
President Vladimir Putin had days earlier recognised the two regions as independent.
The industrial, mainly Russian-speaking regions broke from Kyiv's control in 2014 in fighting that over the next few years claimed more than 14,000 lives.
Russia that year annexed Crimea from Ukraine after a pro-Moscow leader was ousted in a popular uprising and a referendum was held in the southern region on becoming part of Russia.