WASHINGTON -- A former top U.S. Defense Department official has warned that the buildup of NATO troops in Eastern Europe, and Russia's own troop movements along its western borders, are deepening the risk of an accidental outbreak of war.

The comments on June 23 by Ian Brzezinski, deputy assistant secretary of defense for Europe and NATO under former President George W. Bush, came as another former defense official, Derek Chollot, said Russia might feel compelled to ratchet up tensions further if NATO leaders at next month's Warsaw summit make a show of unity.

The two spoke at a U.S. Senate Foreign Relations Committee hearing that previewed some of what's at stake when leaders from the 28-nation alliance meet in the Polish capital on July 8-9.

At the summit, NATO officials are expected to endorse ongoing plans to deploy up to four multinational battalions to Poland and the Baltic states. That coincides with U.S. plans announced earlier this year to begin rotating three armored brigades into Eastern Europe, along with stationing some heavy weaponry and other equipment there.

Those plans have been met with stern warnings from Russia, which has shifted several divisions toward its western border, and with more aggressive maneuvers by Russian fighter jets and bombers.

But the NATO movements have also been met with skepticism within the alliance, most notably in Germany, where the foreign minister said in an interview last week that NATO was engaging in "warmongering."

Chollot, who served as assistant secretary of defense for international security affairs under President Barack Obama, said parliamentary elections scheduled for September in Russia were without a doubt influencing Kremlin policy toward the West.

"I'm worried, and it's not just the election. It's actually how Russia responds to the Warsaw summit. Because if we succeed in having the Warsaw summit be a show of unity, a demonstration that NATO has resolve, and augmentation of the deterrent, Putin may feel the need to respond in some way, to show that he's still willing to do what it takes," he said.

"It's already a pretty perilous period, but I think given the summit, given their election, given perhaps an EU that is mired in an existential crisis...I think it is an opportunity for Russia's adventurism to come back," he said.

NATO recently wrapped up its largest war games since the end of the Cold War, with more than 30,000 troops conducting exercises in parts of Eastern Europe. U.S. naval destroyers patrolling in the Baltic and Black seas, not far from Russia's maritime borders, have been buzzed by Russian jets and shadowed by Russian subs and surface ships.

Those moves, along with the increased presence of regular NATO forces in Eastern Europe and the growing frequency of large-scale snap exercises in Russia, raises the risk of one side misinterpreting the other's intentions, Brzezinski said.

"I'm not worried about intentional attack against NATO. I don't think that's in Putin's plans. But I am worried that his activities do raise, or increase the risk of inadvertent conflict, with all the escalatory dynamics that come with it," he said.

As much as anything, both Challot and Brzezinski said, what NATO lacks right now is a viable form of deterrence to persuade the Kremlin that a credible military response would occur if Russian forces were to stage some sort of intervention in the territory of an alliance member.

The Kremlin will be looking to undermine or discredit any NATO posturing at the Warsaw summit, Brzezinski said, and will be emboldened if there is the perception of a weakening of support for Ukraine.

He said he feared that Putin "will interpret a reinforcement of NATO's eastern frontier with no change in the NATO-Ukraine relationship as a green light to push further into Ukraine."

Challot said sending ground troops into Poland and the Baltic states was significant, but that the force they present needs to be credible to the Russians.

"It's very important that those forces are war-fighting forces, they are forces that can get into the fight in hours and days, not weeks," Challot said. "They are forces that have both the lethal capability, but also the ISR [intelligence, surveillance, reconnaissance] and resupply, to be in that fight. They need air defense as well."

"It’s not present now, and hopefully at Warsaw we will have good news," he added.