(Reuters) - Howmet Aerospace ( HWM ), which supplies parts for planes built by Airbus and Boeing ( BA ), may halt some shipments if they are impacted by tariffs announced by U.S. President Donald Trump, according to a letter seen by Reuters.

Pittsburgh-based Howmet said in the letter to customers that it has declared a force majeure event, a legal practice that allows parties to a contract to avoid their obligations if hit by unavoidable and unpredictable external circumstances.

"Howmet will be excused from supplying any products or services that are impacted by this declared national emergency and/or the tariff executive order," Howmet wrote in the letter.

Howmet declined to comment.

Howmet is a supplier of critical metal components used across the $150 billion jetliner industry.

Boeing ( BA ) and Airbus did not immediately reply to requests for comment on the letter, which three industry sources said went to multiple firms across the aerospace sector.

It appeared to be the first such maneuver by a major aerospace company since the tariff announcement, one of the sources said.

Howmet's rare legal declaration does not mean that supplies will automatically be halted or disrupted but opens the door to the supplier potentially claiming it cannot meet its contracts if impacted by the emergency order.

The letter left room for negotiations over sharing the cost of tariffs, saying Howmet would work with customers "including discussing your interest in alleviating the impact of the Tariff Executive Order on Howmet."

The three industry sources said any successful declaration of force majeure could ripple through the supply chain as companies attempt to pass on the burden.

The move is the latest wrinkle in a hard-hit aerospace supply chain where some companies now face higher costs due to U.S. tariffs on imported aluminum and steel along with fresh duties expected to hit imports from other countries.

On Wednesday, Trump cited trade deficits as a national emergency when he announced tariffs with rates ranging from 10% to around 50%, escalating a dispute that has rattled investors and fed fears of a coming recession.

Aerospace companies typically contest such maneuvers, two of the sources said, adding that the success of Howmet's move would hinge on the degree to which the tariffs were unpredictable, given that Trump had referred to them in his election campaign last year.