NEWPORT, Wales (CNN) -- The spread of brutal Islamist terror across Iraq and Syria and hundreds of deaths in a bloody struggle over Ukraine's future make this a pivotal moment for the NATO alliance, its leaders said Thursday.
"We meet at a crucial time in the history of our alliance," British Prime Minister David Cameron said. "The world faces many dangerous and evolving threats, and it is absolutely clear that NATO is as vital to our future as it has been in our past."
Cameron spoke at the outset of a two-day NATO summit to discuss the alliance's response to threats in the Middle East and Ukraine, as well as Afghanistan's future.
NATO Secretary General Anders Fogh Rasmussen said the alliance "will take important decisions to keep our nations safe, to keep the vital bond between Europe and North America strong and to help build stability in a dangerous world."
The discussions come amid tenuous hopes for peace in Ukraine. A peace plan discussed by Ukraine and Russia is expected to be implemented Friday, Ukrainian President Petro Poroshenko said on his Twitter account Thursday.
A day earlier, Poroshenko's office said that in a phone call, he and Russian President Vladimir Putin had agreed on a process that could lead to a truce between Ukrainian forces and pro-Russian rebels. Putin also presented a seven-point road map to end the conflict in eastern Ukraine, although Russia denies having any involvement in the conflict.
Rebel leaders in eastern Ukraine's Donetsk and Luhansk regions said in a joint statement Thursday that they would be prepared to order a ceasefire as of 3 p.m. Friday "if agreements are achieved and Ukrainian officials sign a plan for a political settlement of the conflict."
The leaders of the self-declared Donetsk and Luhansk People's Republics will take part in talks in Belarus on Friday, the statement said, when they will present their proposals on ensuring compliance with the plan.
Ukrainian Prime Minister Arseniy Yatsenyuk has dismissed Putin's road map as a disguised rescue plan for pro-Russian rebels.
NATO and the United States have also greeted Russia's words with skepticism. Russia annexed Ukraine's Crimean Peninsula in March and is accused of sending its troops into eastern Ukraine in support of pro-Russian rebels, a claim that Moscow denies.
The United States believes Russia now has three to five "battalion task groups" conducting military operations inside Ukraine, according to two U.S. officials. Each group can have as many as a thousand troops, the officials said.
The United States is preparing an additional round of sanctions on top of those already imposed on Russia by U.S. and European governments, Deputy National Security Adviser Ben Rhodes told reporters.
"At the same time, if there is a peaceful de-escalation, that is preferable," he said. "Russia must continue to face costs."
Rasmussen called on Russia to end its disputed annexation of Crimea and withdraw from Ukraine.
"While talking about peace, Russia has not taken one single step to make peace possible," he said.
Earlier, Rasmussen said "Russia's aggression against Ukraine has been a wake-up call."
"It has ... reminded all of us that our freedom, security and prosperity cannot be taken for granted, that some are trying to redraw dividing lines in Europe with force and in blood."
As a result, the alliance must adapt to meet new challenges, Rasmussen said, including the re-emerging threat from Russia, which was a large part of the reason the alliance was formed in 1949.
"We will adopt a readiness action plan that will make our forces faster, fitter and more flexible, ready to address any challenges whenever they come and from wherever they come," Rasmussen said.
NATO members will be urged to prioritize defense, amid concern that defense spending is declining and some member states are not pulling their weight.
U.S. President Barack Obama arrived in Wales for the summit after a visit to Estonia aimed at reassuring nervous Eastern European nations that NATO's support for its member states is unwavering.
In a joint opinion piece published in the Times of London on Thursday, Obama and Cameron warn against isolationism.
"To the east, Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening a sovereign nation state," they wrote. "To the south, there is an arc of instability from north Africa and the Sahel to the Middle East."
The two leaders say that those who argue against addressing these threats fail to understand 21st century reality, adding, "the problems we face today threaten the security of British and American people, and the wider world."
Cameron told CNN ahead of the summit that NATO leaders also would discuss the "poisonous ideology" of Islamist extremism and that NATO members should agree on how to help Middle Eastern nations tackle the ISIS threat.
Any request by Iraq to NATO for aid in fighting ISIS would be "considered seriously," Rasmussen said.
NATO has not yet received such an invitation from Baghdad, Rasmussen said, but help could come in the form of new military training programs such as ones the alliance has held with Iraq in the past.
Cameron declined to rule in or out the possibility of the UK military carrying out airstrikes against ISIS forces, as U.S. forces have done in Iraq.
He said Britain had been "working exhaustively to identify all the people that are potentially involved" in the beheading of U.S. journalists James Foley and Steven Sotloff but declined to share details.
"We share our information with our key allies and (are) making sure we do everything we can to bring these absolutely horrific people to justice," he said.
A British hostage was threatened at the end of the latest execution video, which features a militant with an English accent.
The summit was originally expected to focus on Afghanistan, NATO's biggest overseas commitment of troops, before events elsewhere in the world seized the headlines.
While votes in the contested presidential election are still being audited, Rasmussen said that "time is of the essence" for the Afghan government to finalize a Status of Forces Agreement to protect NATO forces there as they switch to an advisory and training role.
But he did say he was "encouraged" that both candidates in the runoff vote, Abdullah Abdullah and Ashraf Ghani, have agreed on the need for a new agreement.
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