MOSCOW — Ratcheting up pressure on Belarus to join it in a “union state,” Russia rejected pleas from its increasingly balky ally on Friday to provide it with cut-rate oil supplies, insisting that it could not provide any discount.

The Belarusian leader, Aleksandr G. Lukashenko, emboldened by a visit last weekend to his country by Secretary of State Mike Pompeo, pushed hard for cheap energy during what he called a “moment of truth” meeting in Sochi, Russia, with President Vladimir V. Putin. Mr. Putin has used Belarus’s dependence on Russian oil and gas to revive a moribund plan to merge the two countries.

But a senior Kremlin official, Dmitri Kozak, told journalists late Friday that Russia, which temporarily suspended oil deliveries to Belarus last month, could offer only market rates with no discount. He added that negotiations between the two countries on oil sales would continue on a “commercial basis.”

Russia seems to have offered some concessions on supplies of natural gas, with Mr. Kozak saying that favorable terms set in 2019 would continue for the moment. But its firm stand on oil increased pressure on Belarus to accede to demands by Mr. Putin that the two countries implement a long-stalled 1999 plan to unite their two countries in a “union state.”