Ukrainian President Petro Poroshenko expressed guarded optimism Thursday that a peace initiative advanced by Russia can bring a quick end to his country’s months-long conflict.

But Western allies promised an immediate escalation of sanctions against Moscow if the negotiations, planned for Friday, turn out to be “a smoke screen” for further Russian intervention in a war that threatens to unravel the core tenets of European security.

The comments came as NATO leaders gathered at a golf resort in the gentle hill country of southern Wales to hammer out new strategies to confront an arc of crises, from a revived Russian threat to the well-armed and highly organized extremist army that has conquered vast swaths of the Middle East.

After more than a decade in which the world’s most powerful military alliance focused single-mindedly on a distant war — in Afghanistan — NATO leaders find themselves grappling with a panoply of threats that strike far closer to home.

The leaders offered no definitive answers for how they would confront the growing instability. But within the first hours of this two-day summit — which brings together 60 world leaders, including NATO allies and partners — President Obama and British Prime Minister David Cameron indicated they were prepared for a more aggressive Western role.

Writing in the Times of London, the pair rejected calls for “an isolationist approach” and asked their fellow leaders to “summon up the shared resolve that inspired NATO’s founding fathers.”

“Developments in other parts of the world, particularly in Iraq and Syria, threaten our security at home,” they wrote.

But it was Ukraine — and the empty chair of Russia — that dominated the summit’s first day. Russia’s absence was notable — it was the first time in years that Moscow had not been represented at a NATO summit, a sign of the deepening rifts between Moscow and its former Cold War foes.

Even as NATO leaders castigated Russia for its role in stoking a conflict that has claimed more than 2,000 lives, there were glimmers of hope for an end to the fight.

Russian President Vladimir Putin had on Wednesday outlined terms for a cease-fire that seemed to strongly favor the Russian-backed separatists in eastern Ukraine by cementing their territorial gains.

But Poroshenko’s office suggested that he was receptive to a deal, and he said Thursday that he was “carefully optimistic” that talks expected in the Belarusan capital of Minsk could bring the conflict to a close. A cease-fire, he said, could begin as early as noon Friday.

“I’m ready to do my best to stop the war,” Poroshenko said, noting the tremendous toll that the fight has taken on his country. Rebels have made substantial gains in recent weeks and were reported to be moving Thursday on the strategic southeastern port city of Mariupol.

NATO Secretary General Anders Fogh Rasmussen, standing beside Poroshenko in a sign of the alliance’s support, said he backed the Ukrainian president’s decision to pursue diplomacy. But the NATO leader appeared far more skeptical of Putin’s sincerity.

“Previously we have seen similar statements, and they have been a smoke screen for continuing Russian destabilization of Ukraine,” he said. “Based on our experience, we have to be cautious.”

U.S. officials took a similar line, saying they were prepared to impose sanctions coordinated with the European Union if Russia does not halt a military campaign in Ukraine that Western officials say has involved sending an arsenal of weapons and well over 1,000 troops.

In a meeting with the Ukrainian president, Obama and European leaders made clear that “they would continue to support President Poroshenko’s very active diplomacy” in hopes of de-
escalating the conflict, said Benjamin Rhodes, Obama’s deputy national security adviser.

At the same time, Rhodes said the allies were finalizing another package of sanctions, although he declined to give specifics about the kinds of measures under consideration.

“Russia must continue to face costs,” Rhodes said. “If Russia escalates, we stand ready to escalate our pressure. . . . If there can be a peaceful de-escalation, that would be preferable.”

Poroshenko said NATO members had promised on Thursday a variety of assistance for Ukraine, including money, equipment and advice in rebuilding the country’s struggling armed forces.

The alliance itself has ruled out providing arms to Ukraine, although some member countries have suggested that they may contribute lethal equipment on their own.

NATO has said repeatedly that it foresees no direct military intervention in Ukraine, which is a NATO partner but not a member and, therefore, does not benefit from the all-important guarantee of mutual protection. Although Ukraine has said that it would like to join NATO, members are deeply reluctant to agree for fear of provoking Russia and exacerbating the conflict.

Russian Foreign Minister Sergei Lavrov said Thursday that — by asking for NATO membership — Ukraine is making “a blatant attempt to derail all the efforts” toward a peace deal.

While Ukraine took center stage at the summit Thursday, Obama and other U.S. leaders took advantage of the large gathering of allies and partners to try to build a broad coalition against the Islamic State, the jihadist group that has seized vast swaths of territory in Iraq and Syria.

Obama and Cameron urged other NATO members to step up contributions for a coordinated military and political campaign against the Islamic State, warning that the group poses a direct threat to Europe because it has recruited thousands of followers from the continent.

The U.S. military has received assistance from Britain, France and Australia in airlifting humanitarian aid to Iraqi refugees who have fled the group’s advance.

While the Pentagon will almost certainly lean on those countries to join in a possible expanded campaign of airstrikes — which until now have been carried out exclusively by the United States — U.S. officials said they were buttonholing allies for other forms of support. Those include pooled intelligence, expanded training for Iraqi security forces and arms for Kurdish fighters in northern Iraq.

Douglas E. Lute, the U.S. ambassador to NATO, said European countries have become increasingly alarmed by the numbers of their own who have joined the Islamic State and are starting to return home. He cited a May 25 attack on a Jewish museum in Brussels — carried out by a jihadist returning home to Europe from Syria — as a galvanizing moment.

“They understand there are thousands more who are on that path,” Lute said. “They know they’ve got a serious security threat.”

Obama administration officials made clear, however, that they also need to secure strong cooperation from Middle Eastern countries before they can expand military operations against the Islamic State.

Obama was late to a NATO session on Ukraine so he could squeeze in a meeting with King Abdullah II of Jordan to discuss options for combating the Islamist group. He is scheduled to meet with Turkish President Recep Tayyip Erdogan on Friday.

Katie Zezima in Newport and Brian Murphy in Washington contributed to this report.