"The United States enjoys a great friendship and an enduring partnership with our NATO ally Spain," State Department spokeswoman Heather Nauert said in a statement.
"Catalonia is an integral part of Spain, and the US supports the Spanish government's constitutional measures to keep Spain strong and united."
Nauert added that the US and Spain "cooperate closely to advance our shared security and economic priorities."
Catalan lawmakers voted earlier to declare independence, as Madrid vowed in turn to "restore legality" and quash the region's secessionist bid.
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