Spain said on Sunday that it would not join a US-led coalition to protect Red Sea shipping from attacks by Yemen's Huthi rebels.
The Iran-backed Huthis have carried out several strikes on Red Sea shipping in what they say is in support of Palestinians during the Israel-Hamas war in Gaza.
On Thursday, the Pentagon said 20 countries had joined the Washington-led coalition to protect the key shipping lane.
Spain's government has been ambivalent and on Sunday a spokesman for the defence ministry told AFP that Spain "will not participate".
While no reason was given, the Spanish press on Sunday said the decision not to participate was driven by domestic politics.
Pedro Sanchez is in the process of putting together a coalition and needs the support of the radical left party Sumar, which opposes US foreign policy.
The Huthis welcomed Spain's stance, with deputy foreign minister Hussein al-Ezzi saying "we highly appreciate Spain's refusal to be drawn into American and British lies on the issue of maritime navigation".
Pentagon spokesman Major General Pat Ryder said on Thursday that coalition forces would "serve as a highway patrol of sorts, patrolling the Red Sea and the Gulf of Aden to respond to -- and assist as necessary -- commercial vessels that are transiting this vital international waterway".
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