NATO's planned missile defence system is "not targeted against anyone", Spanish Foreign Minister Trinidad Jimenez said Friday a day after Russia criticised Madrid's decision to join the programme.

Spanish Prime Minister Jose Luis Rodriguez Zapatero announced Wednesday during a visit to NATO headquarters in Brussels that Spain had agreed to host four US naval ships equipped with interceptors designed to knock out incoming missiles.

Russia's foreign ministry objected Thursday to the plan, saying the move could end its cooperation with a NATO missile shield.

In a statement, the ministry said the United States had decided to deploy the warships "without collective discussion" and the move raised concerns about a "significant buildup of US missile potential in the European zone".

"If this continues, then the chance created at the (2010) NATO-Russia summit in Lisbon to turn anti-missile defence from an area of confrontation to an area of cooperation may be lost," the statement added.

Leaders of the 28-member NATO alliance gave their backing last year for the Europe-wide ballistic missile shield -- which US officials say is aimed at thwarting missile threats from Iran.

"The shield is a deterrent, it is not offensive, in order to defend ourselves. It is not targeted against anyone," Jimenez said during an interview with Spanish public television.

"Russia was informed by Spain directly before the decision was announced because we have a special relationship. The reaction was the one which Russia traditionally has.

"There was no secrecy, nothing strange, not even a change in policy," she added.

The four ships will be deployed at the US naval base in Rota in southern Spain by 2013.

Washington has also obtained agreements with Poland, Romania and Turkey to host elements of the missile defence shield.

US officials say the Europe-wide ballistic missile shield is needed to protect against threats from Iran and the Middle East.

Moscow has expressed concern that the shield could target Russian intercontinental ballistic missiles used as nuclear deterrents.