Spain agreed Wednesday to bolster NATO's planned missile defence system by hosting four US naval ships equipped with interceptors designed to knock out incoming missiles.

Spanish Prime Minister Jose Luis Rodriguez Zapatero visited NATO headquarters to announce the deal alongside US Defense Secretary Leon Panetta and NATO chief Anders Fogh Rasmussen.

"Spain is a (NATO) member committed to the collective defence of Europe," Zapatero told a news conference, adding that the deal will "also guarantee the defence of our territory and Spanish people".

The Aegis-equipped ships will be deployed at the US naval base in Rota in southern Spain by 2013, Zapatero said.

Panetta said the Rota arrangement was a "critical step" in deploying the shield after a key agreement last month with Turkey to host a sophisticated US radar, and deals with Romania and Poland to host land-based SM-3 interceptors.

"This announcement should send a very strong signal that the United States is still continuing to invest in this alliance," Panetta said.

"We are committed to our defence relationship with Europe even as we face growing budget constraints at home," he said after attending his first meeting of NATO defence ministers since taking office in July.

The move will "give more weight" to the missile shield project and cut costs as the naval ship will not have to transit the Atlantic back to US ports, said a senior US defence official on condition of anonymity.

Rasmussen said he expected more announcements in the coming months, adding he hoped the anti-missile system would be "fully operational by 2018".

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 system, known as the European Phased Adaptive Approach, initially will employ Aegis ships with anti-missile weaponry and eventually expand to include land-based interceptors in Romania by 2015 and Poland by 2018.

One American warship, the USS Monterey, is currently assigned to the missile defence mission, patrolling the Mediterranean.

Under the agreement with Romania, a total of 24 SM-3 interceptors will be deployed at a former airbase in the south of the country, which will host a maximum of 500 US troops.

The United States originally planned to install an anti-missile shield in Poland and the neighbouring Czech Republic. But that plan, which angered Russia as it saw itself as the target for the system, was scrapped by US President Barack Obama in September 2009.

Washington has since reworked the scheme and signed a new treaty with Moscow on reducing strategic nuclear weapons.