“NATO continues to be the linchpin, the cornerstone of our collective defense and US security policy,” Obama said.
“Afghanistan is high on our agenda. Afghanistan is our biggest military operation ever,” NATO Secretary General Jens Stoltenberg told reporters
The three identified suspects behind the twin assaults, which killed 31 and injured 300, have been linked to the Paris attacks last November, underscoring the threat European nations face from the jihadist group.
Medvedev said tensions between Russia and the West have sent the world into a new Cold War.
Syrian peace talks in Geneva earlier this week broke up acrimoniously as long-time Moscow ally President Bashar al-Assad launched a fresh offensive against rebel forces in Aleppo with massive Russian backing.
The announcement sets in motion an accession process that will continue over months before Montenegro formally becomes the 29th member of the alliance.
Turkey PM Davutoglu added that “we hope Russia will reconsider these measures in both our interests”, referring to the sanctions that Moscow imposed after the shooting down of the jet last week.
Obama began US bombing in Iraq and Syria, along with the deployment of military advisers to Iraq, more than a year ago.
Jens Stoltenberg said the Russians have concentrated military forces in Kaliningrad, the Black Sea and the eastern Mediterranean, where they are assisting beleaguered Syrian President Bashar al-Assad.
Poroshenko, who on Tuesday chaired the security council with NATO Secretary-General Jens Stoltenberg in attendance, said he intends to change the decision against joining NATO.
Military jets and helicopters flew over the city centre until the early hours of Saturday morning as security forces locked down the area around the attacks.
“The security of the alliance is indivisible,” ambassadors from all 28 NATO nations declared in a joint statement after the meeting.
NATO fears intense fighting during summer in eastern Ukraine accusing Moscow of supporting Russian separatist, a charge denied by Moscow.
NATO allies deploy troops, gather arms, schedule military training and upgrade bases in preparation of a possible conflict with Russia and other threats.
Carter called for NATO nations to stand united against an aggressive Russia but insists US is not seeking a conflict with Moscow.