Washington tries to check Hungary's drift into Kremlin orbit

(Repeats to additional clients without changes)

* MOL may sell stake in Croatia energy firm to Russian buyer

* U.S. officials lobbying to stop sale happening

* Washington imposed visa ban on six Hungarians

* Hungary pulled out of Western drive to give Ukraine gas

By Zoran Radosavljevic and Krizstina Than

ZAGREB/BUDAPEST, Oct 31 (Reuters) - The United States is mounting a diplomatic offensive to stop Hungary selling a stake in a Croatian energy firm to Russia, part of what Western powers see as Budapest's dangerous drift into Moscow's orbit.

The U.S. government has already taken the highly unusual step of blacklisting six people with ties to the government in Hungary, a NATO ally and European Union member, from entering the United States, accusing them of involvement in corruption.

U.S. officials say that demarche was the result of growing exasperation with Hungarian Prime Minister Viktor Orban, who has pushed judges into retirement, imposed heavy levies on foreign banks, and this week sparked huge protests with a proposal to tax Internet use.

But Washington is particularly preoccupied about a growing closeness between Hungary and the Kremlin over energy that could undermine Western attempts to isolate Russian leader Vladimir Putin over his intervention in Ukraine.

Since September, Hungary has stopped pumping natural gas to Ukraine, effectively pulling out of an EU-backed effort to support Kiev in the face of a Russian energy blockade, and it has renewed a commitment to build a Kremlin-backed pipeline for Russian gas, South Stream, that Washington and Brussels oppose.

U.S. officials are now worried that Hungarian energy firm MOL will sell its 49 percent stake in INA, Croatia's biggest energy company, to a Russian firm, possibly state-owned Gazprom. The Hungarian state has a 24.7 percent stake in MOL.

A State Department official responsible for energy security asked a U.S. senator who was visiting Europe to make a detour to Croatia last weekend to lobby the government there on the issue.

The State Department official, Amos J. Hochstein, also met Hungarian Foreign Minister Peter Szijjarto in Washington earlier this month. A State Department official told Reuters they had a "productive meeting" talking about MOL's stake, South Stream, and Hungarian gas deliveries to Ukraine, among other issues.

Western diplomats in the region have confirmed the United States is worried about a possible sale to Gazprom, a firm they describe as a tool of Kremlin policy.

A Gazprom takeover would give the Russian company a strategic foothold inside the European Union, which is already its biggest customer for natural gas.