The meeting, being held at a golf resort just outside of the Welsh city of Newport, kicked off with a ceremony in which NATO Secretary General Anders Fogh Rasmussen and the national leaders paid tribute to soldiers killed while serving in NATO missions abroad.
Soldiers from the member and allied states carried their respective flags into the large conference room where the leaders are meeting, before they held a minute of silence to remember the fallen soldiers.
The summit had originally been meant to focus mainly on Afghanistan, where NATO troops are to wind down their combat mission by the end of the year. However, due to the developments of the past few months, the crisis in eastern Ukraine and the advance of "Islamic State" militants in Syria and Iraq are expected to dominate much of the agenda.
Prior to the start of the talks, Rasmussen, who described this as "one of the most important summits in the history of our alliance," accused Russia of ongoing involvement eastern Ukraine despite presenting a peace plan.
"We are faced with a dramatically changed security environment. To the east, Russia is attacking Ukraine," Rasmussen said. He also said that as long as the Kremlin continued on its current path, a seven-point peace plan presented by Russian President Vladimir Putin on Wednesday was virtually meaningless.
"What counts is what is actually happening on the ground," the NATO boss said. "And we are still witnessing, unfortunately, Russian involvement in destabilizing the situation in eastern Ukraine."
US President Barack Obama and British Prime Minister David Cameron also sharply criticized Moscow.
"Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening and undermining a sovereign nation state," the two leaders wrote in a joint statement released on Thursday.
Earlier, Russian Foreign Minister Sergey Lavrov accused Ukraine of seeking to undermine efforts to make peace with its bid to join NATO.
Even without NATO membership, Poroshenko was in Newport on Thursday, where he held talks with a number of leaders, including Obama, Cameron and German Chancellor Angela Merkel. He was to meet with all 28 leaders later in the day.
'Islamic State' also on the agenda
As for 'Islamic State' fighters who have taken large swathes of Syria and northern Iraq in recent months, Rasmussen said the international community "has an obligation to stop the Islamic State from advancing further." At the same time, he noted that so far, the alliance hadn't received any request for help.
"I'm sure that if the Iraqi government were to forward a request for NATO assistance, that would be considered seriously by NATO allies," he said.
pfd/sb (AFP, Reuters , AP, dpa)