BERLIN, May 5, 2025 (BSS/AFP) - Germany's far-right AfD party said on Monday it was suing the domestic intelligence agency for labelling it a "right-wing extremist" party, a designation that sparked a row between Berlin and Washington.

An spokesman for the Alternative for Germany (AfD) said the party had lodged a challenge against the BfV agency's designation with the administrative court in the western city of Cologne.

The BfV said on Friday it considered the AfD a "confirmed" right-wing extremist group, citing the "xenophobic, anti-minority, Islamophobic and anti-Muslim statements made by leading party officials".

The classification will help authorities seek greater powers to monitor the party with phone taps and undercover agents, and has fuelled fresh calls for an outright ban on the party.

Members of US President Donald Trump's administration were quick to leap to the AfD's defence in the wake of the BfV's decision.

US Vice President JD Vance described the AfD -- which like Trump has campaigned against immigration -- as "the most popular party in Germany".

The AfD came second in Germany's federal elections in February with just under 21 percent, behind the conservative CDU-CSU which won nearly 29 percent.

US Secretary of State Marco Rubio called the intelligence agency's move "tyranny in disguise" and said "Germany should reverse course".

The German foreign ministry took the unusual step of replying directly to Rubio on X to say: "This is democracy." It stressed the BfV's move was "the result of a thorough and independent investigation to protect our constitution".