Brussels, Belgium - Elon Musk's X social media platform said Friday that it had filed an appeal with the European Union's top court against a 120-million-euro ($142 million) fine for breaking its digital content rules.

The European Commission imposed the penalty in December for violations, including breaching its transparency obligation, triggering an angry reaction in the US.

X said it filed an appeal at the General Court of the EU challenging the fine by the commission, which acts as the EU's digital watchdog.

The fine was the first ever under the bloc's landmark Digital Services Act (DSA), which has come under fierce attack in the US, including claims that it allows censorship.

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X on Friday denounced what it called the EU's "incomplete and superficial investigation."

Its global government affairs team said on the platform that the EU's probe included "grave procedural errors, a tortured interpretation of the obligations under the DSA, and systematic breaches of rights of defence and basic due process requirements suggesting prosecutorial bias."

"This landmark case is the first judicial challenge to a DSA fine," it added.

An EU spokesman told AFP the commission was aware of the appeal, and "is ready to defend its decision in court."