The Office of U.S. Trade Representative proposed Monday an additional $4 billion in European Union tariffs.

The proposal builds on an initial list the trade office published in April, adding more goods facing potential tariffs.

The tariffs are in response to EU aircraft subsidies, and the listed products are valued at $21 billion.

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The Office of U.S. Trade Representative said the additions were in response to "public comments and additional analysis," without expanding on its reasoning.

The proposal was first reported by Reuters.