By Arshad Mohammed

WASHINGTON (Reuters) - The United States on Friday imposed sanctions on a Crimea-based gas company, Chernomorneftegaz, effectively putting it off limits to Russia's state-controlled Gazprom, which was expected to bid for a stake in the company.

The move, along with penalties on six Crimean separatists and a former Ukrainian official, is the third round of U.S. sanctions since the Ukraine crisis erupted and lays down a harder line ahead of talks among U.S., Russian, Ukrainian and EU officials in Geneva on Wednesday.

Russian forces took over Crimea last month and Moscow annexed the Ukrainian region on March 18, angering Western powers who say Russia has massed forces on its border with Ukraine, possibly as a prelude to seizing more of the country.

Russia denies having such plans.

While there is no talk of the West going to war over Ukraine, the Western response has included economic sanctions, efforts to bolster the defenses of other European nations who fear Russia, and strong condemnation of Moscow's actions.

In a statement, the U.S. Treasury Department named the individuals placed under sanctions as separatists Pyotr Zima, Aleksei Chaliy, Rustam Temirgaliev, Yuriy Zherebtsov, Mikhail Malyshev, and Valery Medvedev and former Ukrainian official Sergey Tsekov.

U.S. officials said the move aimed to make it impossible for Gazprom to have dealings with Chernomorneftegaz.

On April 1, Russia's energy minister said Gazprom would pay for the construction of an undersea gas pipeline to Crimea and would take part in a tender on the privatization of the local energy company.

"We are saying to the world, with some teeth, you are not going to invest in Crimea with impunity," said a U.S. official.

The official, who spoke on condition of anonymity because of the sensitivity of the diplomacy on Ukraine, added that if Gazprom were to acquire part of Chernomorneftegaz or to deal with it, the Russian oil company could face sanctions.

(Reporting By Arshad Mohammed; Editing by Meredith Mazzilli and Mohammad Zargham)