Czech power giant CEZ on Tuesday asked for better bids from the US and Russian groups that are vying to build two new units at its southern Temelin nuclear plant.

"We certainly expect a major improvement in all respects," CEZ spokesman Ladislav Kriz told Czech television.

US industrial giant Westinghouse is up against the MIR-1200 consortium, made up of Russia's Atomstroiexport and Gidropress and the Czech Skoda JS.

France's Areva was eliminated from the running in October but has appealed the decision.

The winner of the contract worth an estimated 200 to 300 billion koruna (8 to 12 billion euros, $10 to 15 billion) is due to be announced in autumn.

Talks with the groups started early this month, following a preliminary assessment of the bids in late March, Kriz said.

"In our opinion, the bidders didn't say their last word in their initial bids," he added.

The units due to come online in 2025 aim to raise the share of nuclear power in the Czech energy mix to 50 percent from the current 30 percent.

Planned in the communist era and launched in 2000, the Temelin facility has been repeatedly criticised by Austria regarding safety concerns.

The plant, which is located around 60 kilometres (35 miles) from the Austrian border, has two Russian-type VVER pressurised-water reactors, each with an output of 1,000 megawatts.

Locals have dubbed it "Disneyland" on account of the colourful lights that illuminate it at night.