In a statement by a spokeswoman after Republicans and Democrats in the US Congress reached a deal that could see new legislation pass, the European Commission warned of possibly "wide and indiscriminate" "unintended consequences", notably on the EU's efforts to diversify energy sources away from Russia.
Germany has already warned of possible retaliation if the United States moves to sanction German firms involved with building a new Baltic pipeline for Russian gas.
EU diplomats are concerned that a German-US row over the Nord Stream 2 pipeline being built by Russia's state-owned Gazprom could complicate efforts in Brussels to forge an EU consensus on negotiating with Russia over the project.
"We highly value the unity that is prevailing among international partners in our approach towards Russia's action in Ukraine and the subsequent sanctions. This unity is the guarantee of the efficiency and credibility of our measures," the Commission said in its statement.
"We understand that the Russia/Iran sanctions bill is driven primarily by domestic considerations," it went on, referring to a bill passed in the US Senate last month and to which lawmakers said on Saturday they had unblocked further obstacles.
"As we have said repeatedly, it is important that any possible new measures are coordinated between international partners to maintain unity among partners on the sanctions that has been underpinning the efforts for full implementation of the Minsk Agreements," the Commission said, referring to an accord struck with Moscow to try to end the conflicts in Ukraine.
"Sanctions are at their most effective when they are coordinated. Currently our sanctions regimes are coordinated. As a result their impact on the ground is increased and through coordination we are able to avoid surprises, manage potential impact on our own economic operators and address collectively efforts to circumvent such measures. Unilateral measures would undermine this," the Commission said.
"We therefore call on the US Congress/authorities to engage with the partners, including the EU, to ensure coordination and to avoid any unintended consequences of the measures discussed."
(Reporting by Alastair Macdonald; Editing by Catherine Evans)
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