The US State Department lowered the EU mission's diplomatic status from member state to international organization last month.
The snub was first reported Tuesday by German broadcaster Deutsche Welle, Presstv Reported.
"It's clearly political and, frankly, amateur and unprofessional," said one diplomat from a non-EU country. "I'm putting it politely."
The EU learned of the diplomatic downgrade during the December 5 funeral of former US President George H.W. Bush, when EU Ambassador David O'Sullivan was among the last diplomats to be called to pay respects.
"Every administration has the right to review this order but why now, two years in?" an EU official said. "What for us is remarkable is that we were not notified," the official continued. "For a diplomatic move like that, you should at least tell the organization involved."
"Europeans have become accustomed to learning about US policy decisions in the newspaper,” said Julianne Smith an adjunct fellow at the Center for a New American Security. “That's not how alliances work, and that's not how past administrations have treated allies. Europeans are growing tired of the constant surprises."
The snub is just the latest irritant in US-EU relations since the Trump administration took office. The US president has often shown his disdain for the EU and has backed Britain’s planned departure from the bloc.
Under Trump, the US has sparred with the EU on a number of issues, including tariffs, the Iran nuclear agreement and support for the Paris climate accord.
In a speech in Brussels in December, US Secretary of State Mike Pompeo cast doubt on the EU’s commitment to its citizens, drawing a rare rebuke from the European Commission, the bloc’s executive.