U.S. Trade Representative Robert Lighthizer on Monday expressed disappointment about the European Union's decision to impose tariffs on U.S. planes and other goods, but refrained from threatening to respond with further U.S. action.

The EU on Monday said it would impose tariffs on up to $4 billion of U.S. planes and other imports in retaliation for U.S. subsidies for Boeing, following a green light given by the World Trade Organization last month.

Lighthizer last month had said any move by Brussels to impose tariffs would force a U.S. response, but a brief statement issued late on Monday contained no such threat, and his office said the two sides were in negotiations to resolve the longstanding dispute.

"The United States is disappointed by the action taken by the EU today," he said. "The EU has long proclaimed its commitment to following WTO rules, but today’s announcement shows they do so only when convenient to them."

Washington argues that the alleged subsidy - a Washington State tax break - was repealed seven months ago, removing any legal basis for the EU measures.

Brussels says only the WTO can determine whether members have complied with its rulings.

EU officials underscored their desire to settle the 16-year dispute with the United States, arguing that China and other competitors are the chief beneficiaries.