NATO leaders united Saturday behind a "hard-headed" policy of deterrence and dialogue with Russia after launching the alliance's biggest military revamp since the Cold War to counter a resurgent Moscow.

Alliance chief Jens Stoltenberg said alliance leaders "stand together" on Russia, agreeing at a summit in Warsaw to bolster its eastern flank after Moscow's annexation of Crimea and the Ukraine conflict.

Russian President Vladimir Putin has opposed NATO's decision to put four battalions in Poland and the Baltic states, seeing the expansion into Moscow's Soviet-era backyard as a direct security threat.

"The alliance is united, we stand together," Stoltenberg said when asked about the leaders' talks on Russia. "The united message is that defence and dialogue are what our relationship is based on."

Around 200 anti-war activists defied heavy security in central Warsaw to protest against the summit, carrying banners saying "Yes to Peace, No to NATO", AFP journalists saw.

Unity was the buzzword of the two-day summit in the Polish capital after Britain's shock vote to quit the European Union raised questions about its future role as a nuclear armed global power.

Prime Minister David Cameron reassured his peers that Britain was committed to the alliance and announced a parliamentary vote on July 18 on revamping Britain's Trident nuclear deterrent to back that up.

"The nuclear deterrent remains essential in my view not just to Britain's security but as our allies acknowledge here today to the overall security of the NATO alliance," Cameron told a news conference.

Cameron said Britain fully backed the measures adopted by NATO in response to the Ukraine crisis "but we must also engage in a hard headed dialogue with Russia".

- Underlying divisions -

NATO's two-track strategy reflects underlying divisions in the bloc, with calls from France and Germany to avoid a Cold-War style stand-off when Moscow's help is needed on issues such as terrorism.

The United States and European Union have both imposed sanctions on Moscow over the Ukraine crisis but in Europe in particular there are growing calls for them to be scaled back.

French President Francois Hollande appeared to offer an olive branch to Moscow, saying on Friday Russia was neither adversary nor threat but a partner.

Leaders will discuss Ukraine with President Petro Poroshenko later Saturday.

Stoltenberg announced earlier this week that the alliance would hold a fresh meeting with Russia on Wednesday as a gesture of the West's openness and good faith.

Eastern European states have previously warned against easing the pressure, but NATO's unprecedented new measures have begun to reassure them.

Lithuanian President Dalia Grybauskaite, a sharp critic of Putin, said a stronger NATO would be better able to talk to Russia.

"I am softening" on the need for dialogue, Grybauskaite told AFP Saturday.

Putin is unlikely to miss the symbolic importance of the summit being held in Warsaw, the birthplace of the Soviet-era Warsaw Pact, once NATO's adversary.

Moscow bitterly opposes NATO's expansion into its Soviet-era satellites, seeing it as part of an attempt to surround it.

Russia is even more critical of the Ballistic Missile Defence system the United States is building to counter missile threats from Iran or the Middle East, which NATO declared initially operational on Friday.

- Afghanistan pledges -

NATO leaders meanwhile confirmed pledges to fund Afghanistan security forces until 2020, to combat Taliban rebels who are putting the Kabul government under intense pressure.

Stoltenberg said NATO will keep troops in Afghanistan until 2017 under its train and advise Resolute Support mission but could not say when the alliance's longest military engagement might end.

US President Barack Obama was forced this week to slow the US pullout saying 8,400 US troops will now remain in the war-ravaged country into next year.

The next NATO summit will be held in 2017 at the alliance's new $1.2 billion headquarters in Brussels, Stoltenberg added.

At the Warsaw protest, hundreds of activists chanted slogans against the NATO build-up, watched over by around 100 police officers.

"I wish NATO had disappeared with the Warsaw Pact," said French environmental activist Gerard Levy.

Protester Ilya Bydrartskis, from Russia, added: "I am against imperialism... NATO's forces are stronger than Russia's but I am against Putin too!"