BRUSSELS - NATO chief Anders Fogh Rasmussen called on the military alliance Thursday to transform into a modern force against new threats by investing in a missile defence system despite times of austerity.

Rasmussen made his plea at the start of a rare meeting of defence and foreign ministers at NATO headquarters to shape the 28-nation alliance's vision for the next decade amid a tough fight in Afghanistan and budget cuts at home.

"NATO's core mission to protect the 900 million citizens of NATO countries from attack must never change, but it must be a modern defence against modern threats," he said.

Rasmussen called on the alliance to press ahead with plans to create an anti-missile shield for Europe in the face of threats from ballistic missiles from hostile states.

"The threat is clear, the capability is clear and the costs are manageable," he told defence chiefs ahead of the meeting.

U.S. Defence Secretary Robert Gates told reporters there was "broad agreement" on missile defence plans and that the cost of linking NATO members into a common network was "relatively modest."

France and other nations, however, want more details about how much the system would cost and how it would work, diplomats said.

France, a nuclear-armed power, is also at odds with Germany, which is pushing for missile defence as a subsitute to nuclear deterrence, diplomat said.

"We believe that, on substance, the missile shield is a good idea," German Defence Minister Karl-Theodor zu Guttenberg told reporters ahead of the meeting.

"But we also believe that (nuclear) disarmament can and must play an important role."

The ministers gathered are to discuss a draft of NATO's new "strategic concept," a text revised every decade that serves as a mission statement for the alliance.

Afghanistan was not on the agenda but loomed large over the meeting. Three NATO soldiers were killed in a bomb attack in Afghanistan on Thursday, bringing to 584 the total number of personnel killed so far this year.

The 11-page strategic concept, drafted by Rasmussen, has not been made public but it is expected to touch on 21st century threats including cyber attacks, missiles from "rogue" states, terrorism and Somali piracy.

The document will be endorsed by NATO leaders at a summit in Lisbon next month, replacing a document written in 1999, two years before the September 11 attacks on the United States sparked the war in Afghanistan.

The ministers were also discussing proposals to reform NATO's command structure and cut the number of agencies, an idea pushed by Rasmussen to make the 28-nation alliance "leaner and more efficient."

At the same time, Rasmussen and U.S. officials have urged allies to resist cutting too deeply into their own national defence budgets.

"My worry is that the more our allies cut their capabilities, the more people will look to the United States to cover whatever gaps that are created, at a time when we're facing" fiscal pressures, Gates said.