NATO needs to develop an anti-missile defence system as a deterrence, the alliance chief said Friday, while seeking to assure Moscow that the organisation posed no threat to Russia.

"We must develop an effective missile defence," NATO Secretary General Anders Fogh Rasmussen told an international conference in the Polish capital.

"In the coming years we will probably face many more countries and possibly even some non-state actors armed with long-range missiles and nuclear capabilities," he said.

Rasmussen also insisted later during a press conference that "a nuclear capability will remain an essential part of a credible deterrence in the future."

"I share the great vision of a world free of nuclear weapons.

"But as long as we do have nuclear weapons on earth and as long as we know there are countries and non-state actors that aspire to acquire such nuclear capacities, I think we should have a nuclear capacity as part of our deterrent policy."

Rasmussen told the conference that a system for protection against missiles should be part of NATO's policy of deterring threats.

"Deterrence works against rational actors but not all actors that we will have to deal with in the future will be rational.

"That's why deterrence and defence need to go together and why we have the obligation to look into the missile defence options," he said.

Anti-missile defence systems already in place within the NATO alliance fall under a US shield that has missile interceptors in the United States, Greenland and Britain.

Plans for it to be extended into eastern Europe have raised serious concern in Russia.

Turning to Russia, Rasmussen said Moscow's policy towards Georgia are fuelling "profound concerns" in NATO countries.

Russia and Georgia fought a brief war in August 2008 over the breakaway Georgian region of South Ossetia, which Russia later recognised as an independent state along with another rebel region, Abkhazia.

Rasmussen also said "Russia sent a wrong kind of signal by conducting military exercises that rehearse the invasion of a smaller NATO member," referring to Russian and Belarus war games in September on Poland's border.

in February, the Kremlin published a strategy paper listing first among "chief outside military threats" the fact that NATO is attempting to "globalise its functions in contravention of international law."

"Russia's new military doctrine does not reflect the real world. It contains a very outdated notion about NATO and the role of NATO," Rasmussen said.

"Let me stress NATO is not a threat to Russia and NATO will never invade Russia nor do we consider Russia a threat to NATO."

He stressed that "a much improved relationship between NATO and Russia would be the best reassurance of all our nations."

Polish Foreign Minister Radoslaw Sikorski told the press conference that "Secretary General Rasmussen's effort to re-engage Russia are most welcome."

"Central Europe wants NATO to develop relations with Russia as a partner pragmatically and with full respect to the legitimate security concerns of both sides," he added.