WARSAW (Reuters) - US President Barack Obama urged Nato leaders on Friday (July 8) to stand firm against a resurgent Russia over its seizure of Crimea from Ukraine, saying Britain's vote to leave the European Union should not weaken the Western defence alliance.

In an article published in the Financial Times newspaper as he arrived for his last summit of the North Atlantic Treaty Organisation before he leaves office in January, Obama said America's "special relationship" with Britain would survive the referendum decision he had warned against.

"The special relationship between the US and the UK will endure. I have no doubt that the UK will remain one of Nato's most capable members," he said, but noted that the vote raised significant questions about the future of EU integration.

The 28-nation EU will formally agree to deploy four battalions totalling 3,000 to 4,000 troops in the Baltic states and Poland on a rotating basis to reassure eastern members of its readiness to defend them against any Russian aggression.

Host nation Poland set the tone of mistrust of Russia. Its foreign minister, Witold Waszczykowski, told a pre-summit forum:"We have to reject any type of wishful thinking with regard to a pragmatic cooperation with Russia as long as it keeps on invading its neighbours."

Obama was more diplomatic, urging dialogue with Russia, but he too urged allies to keep sanctions on Moscow until it fully complies with a ceasefire agreement in Ukraine, and to help Kiev defend its sovereignty.

Ukraine is not itself a member of Nato.

"In Warsaw, we must reaffirm our determination - our duty under Article 5 of the North Atlantic Treaty - to defend every Nato ally," Obama said. "We need to bolster the defence of our allies in central and eastern Europe, strengthen deterrence and boost our resilience against new threats, including cyber attacks."

Estonia, Latvia, Lithuania and Poland - all Nato members - have requested a permanent Nato presence. They fear Moscow will seek to destabilise their pro-Western governments through cyber attacks, stirring up Russian speakers, hostile broadcasting and even territorial incursions.

Critics say the Nato plan is a minimal trip wire that might not deter Russian action.

The head of Nato's military committee, Czech General Petr Pavel, said Russia was attempting to restore its status as a world power, an effort that includes using its military. "We must accept that Russia can be a competitor, adversary, peer or partner and probably all four at the same time," he said.

The Kremlin said it was absurd for Nato to talk of any threat coming from Russia and it hoped "common sense" would prevail at the Warsaw summit. Moscow was and remains open to dialogue with Nato and is ready to cooperate with it, Kremlin spokesman Dmitry Peskov said in a conference call with journalists.

Russia often depicts Nato as an aggressor, whose member states are moving troops and military hardware further into former Soviet territory, which it regards as its sphere of influence.

Russian President Vladimir Putin made several gestures aimed at showing a cooperative face before the summit. At the same time, Moscow highlighted its intention to deploy nuclear-capable missiles in Kaliningrad, a Russian enclave between Poland and Lithuania.

Putin agreed to a meeting of the Nato-Russia Council next week, the second meeting this year of a consultation body that was put on ice after Moscow's seizure of Crimea in 2014. Russia allowed a UN resolution authorising the EU to intercept arms shipments to Libya in the Mediterranean, and Putin talked by telephone with Obama in the run-up to the Nato meeting.

However, a White House spokesman said they reached no agreement on cooperation in fighting Islamic State militants in Syria during that call on Wednesday.

BRITAIN

Outgoing British Prime Minister David Cameron, who said he will resign after losing the referendum on EU membership last month, will seek to emphasise an active commitment to Western security at his final Nato summit, to offset any concern about Europe's biggest military spender leaving the EU.

The first item on the summit agenda was the signing of an agreement between the EU and Nato on deeper military and security cooperation.

The US-led alliance is also expected to announce its support for the EU's Mediterranean interdiction operation. Nato already supports EU efforts to stem a flood of refugees and migrants from Turkey into Greece, in conjunction with an EU-Turkey deal to curb migration in return for benefits for Ankara.

Obama and the other Nato leaders will have a more unscripted discussion of how to deal with Russia over dinner in the same room of the Polish Presidential Palace where the Warsaw Pact was signed in 1955, creating the Soviet-dominated military alliance that was Nato's adversary during the Cold War.

Nato Secretary-General Jens Stoltenberg sought to balance the new military deployments and air patrols close to Russia's borders by stressing the alliance would continue to seek"meaningful and constructive dialogue" with Moscow.

"We don't want a new Cold War," he told reporters. "The Cold War is history and it should remain history."

Turkish President Tayyip Erdogan told reporters before leaving Ankara to attend the summit that Nato also needed to adapt to do more to fight a threat from Islamic State militants, who were accused of last week's deadly attack on Istanbul airport.

"As we have seen from the terrorist attacks first in Istanbul and then in Iraq and Saudi Arabia, international security is becoming more fragile," Erdogan said.

"The concept of a security threat is undergoing a serious change. In this process, Nato needs to be more active and has to update itself against the new security threats," he said.