The European Union on Tuesday warned that it would retaliate if the United States slaps tariffs on EU exports.

The bloc’s warning comes as a 15-year-long Airbus-Boeing row continues to escalate.

The World Trade Organization is expected to allow the imposition of billions of dollars in tariffs by the U.S. against the E.U. as early as Wednesday.

"We are determined to pursue a positive trade agenda with the US, but ready to firmly defend our interests when and if necessary within WTO rules." said Ville Skinnari, the trade minister from Finland, which holds the EU's six-month rotating presidency.

The Airbus-Boeing row was sparked in 2004 by accusations from the U.S. that the U.K., France, Germany and Spain were providing illegal subsidies and grants to support the production of a range of Airbus products.

The E.U. countered the allegations by accusing Boeing of having received $19.1 billion worth of prohibited subsidies from 1989 to 2006 from various branches of the US government.

The E.U. in July proposed a truce to end the protracted legal battle in which both sides would admit fault and forge ways to curtail airline subsidies.

"Until the very last moment, we are hoping that we can start that dialogue, freezing our respective tariffs and enter that dialogue with the US." said EU Trade Commissioner Cecilia Malmstrom after talks in Brussels with European trade ministers.

"So far we have not received any positive response on that." she added.

The relationship between the U.S. and the E.U. has been has been strained by the Donald Trump administration’s “America First” foreign policy.

In May 2019, President Trump said the E.U. treats the U.S. “worse than China” does on trade.