BRUSSELS (Reuters) - The European Union warned on Wednesday that it was ready to act within days to counter proposed new U.S. sanctions on Russia, saying they would harm the bloc's energy security.

Sanctions legislation overwhelmingly approved by the U.S. House of Representatives on Tuesday has angered EU officials: they see it as breaking transatlantic unity in the West's response to Moscow's annexation of Crimea from Ukraine in 2014 and its support for separatists in eastern Ukraine.

Brussels also fears the new sanctions will harm European firms with connections to Russia, and oil and gas projects on which the EU is dependent.

"The U.S. bill could have unintended unilateral effects that impact the EU's energy security interests," EU chief executive Jean-Claude Juncker said in a statement issued after a meeting at which European commissioners were united in their views, according to a senior EU official.

"If our concerns are not taken into account sufficiently, we stand ready to act appropriately within a matter of days. 'America First' cannot mean that Europe's interests come last," he said, mentioning President Donald Trump's guiding slogan.

A EU document prepared for the commissioners, seen by Reuters, laid out the EU's plans to seek "demonstrable reassurances" that the White House would not use the bill to target EU interests.

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The bloc, it says, will also prepare to use an EU regulation allowing it to defend companies against the application of extraterritorial measures by the United States.

If diplomacy fails, Brussels plans to file a complaint at the World Trade Organization. "In addition, the preparation of a substantive response that would deter the U.S. from taking measures against EU companies could be considered," it says.

However, most measures taken by Brussels would require approval from all 28 EU member governments, which could expose potential differences in individual nations' relations with Moscow and Washington.

Despite changes to the U.S. bill that took into account some EU concerns, Brussels said the legislation could still hinder upkeep of the gas pipeline network in Russia that feeds into Ukraine and supplies over a quarter of EU needs. The EU says it could also hamper projects crucial to its energy diversification goals, such as the Baltic Liquefied Natural Gas (LNG) project.

The new sanctions target the disputed Nord Stream 2 project for a new pipeline running from Russia to Germany under the Baltic Sea. But the EU note says: "the impact would in reality be much wider."

A list prepared by the EU executive, seen by Reuters, shows eight projects including those involving oil majors Anglo-Dutch Shell, BP and Italy's Eni that risk falling foul of the U.S. measures.

Voicing frustration at the fraying in the joint Western approach to Moscow, Juncker said "close coordination among allies" was key to ensuring that curbs on business with the Russian energy, defense and financial sectors, imposed in July 2014, are effective.

EU sources said Juncker told Commissioners the risk to EU interests was collateral damage of a U.S. domestic fight between Trump and U.S. lawmakers.

It was unclear how quickly the U.S. bill would reach the White House for Trump to sign into law or veto. The bill amounts to a rebuke of Trump by requiring him to obtain lawmakers' permission before easing any sanctions on Moscow.

Rejecting the legislation - which would potentially stymie his wish for improved relations with Moscow - would carry a risk that his veto could be overridden by lawmakers.

INDUSTRY CONCERNS

European energy industry sources voiced alarm at the potentially wide-ranging damage of the new U.S. measures.

"This is pretty tough," one industry source told Reuters. "We are working with EU officials to see what safeguards can be anticipated to protect our investment and give us certainty."

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Five Western firms are partnered with Russia's Gazprom in Nord Stream 2: German's Wintershall [WINT.UL] and Uniper, Anglo-Dutch Royal Dutch Shell, Austria's OMV and France's Engie.

But EU officials warn the U.S. measures would also hit plans for the LNG plant on the Gulf of Finland in which Shell is partnering with Gazprom.

The EU document shows they might jeopardize Eni's 50 percent stake in the Blue Stream pipeline from Russia to Turkey as well as the CPC pipeline, carrying Kazakh oil to the Black Sea, involving European groups BG Overseas Holdings, Shell and Eni .

It further warns that BP would be forced to halt some activities with Russian energy major Rosneft.

(Additional reporting by Philip Blenkinsop and Robin Emmott; editing by Mark Heinrich and Mark Trevelyan)