"Simply put, if they range several tens of thousands of conventional armed forces alongside the border of a NATO member nation like they've done to Ukraine, in a coercive manner, that member nation needs to know that NATO will put the appropriate defensive presence on their territory to offset that pressure for as long as it is required," Sir Adrian Bradshaw, a British army general, said in an Associated Press interview at the Pentagon.
Bradshaw said there is no sign that Russia intends to put military pressure on any NATO country.
"We're not saying they're planning to do it, we're not saying there are any indications that they've got a NATO nation in mind right now," Bradshaw said.
"But what we're saying is that against that possibility we just need to have our forces at the right responsiveness."
NATO already has a designated crisis-response group, known as the NATO Response Force, but Bradshaw said it lacks a "very high readiness" element of combat forces.
He said NATO's military leaders believe such a quicker-reaction force is appropriate, but that creating it would be up to the alliance's political leaders, who meet in Wales in early September to discuss Ukraine and other issues.
Ukraine is not a NATO member but has a partnership arrangement, as does Russia.
So the U.S. and other alliance members would be under no treaty obligation to defend Ukraine if it were invaded by Russia.
Still, the U.S. and other countries are keeping a close eye on the capabilities of Russian forces along the Ukrainian border and have repeatedly called on Moscow to move its forces away from the border.
At a news conference Friday, the Pentagon's press secretary, Navy Rear Adm. John Kirby, said the U.S. estimates that Russia has more than 10,000 troops along that border.
He described them as "very capable, very ready" forces and said they are positioned within 50 kilometers, or about 30 miles, of the border.
Kirby said that puts them closer to Ukrainian territory than when Russia began sending large numbers of troops to the area last spring.
"Just as critically, the Russian government continues to support the separatists in eastern Ukraine," Kirby said.
"So, taken as a whole, their activities continue to destabilize the region."
The British general said NATO cannot rule out future surprises from Russian President Valdimir Putin.
"Putin, in doing what he's done in Ukraine, he's mounted the tiger of Russian nationalism and his popularity ratings are sky high," Bradshaw said.
"That begs the question where he might go, what he might feel he needs to do should his popularity in the future start to decline. This is dangerous behavior."
Bradshaw called Russia's intervention in Ukraine, including its annexation of the Crimea region and its support for separatist forces in eastern Ukraine, reminiscent of Europe in the 1930s, when Nazi Germany was on the march.
"This is not the way that we expect international relations to be conducted in Europe in the 21st century, and it represents a change of the security paradigm in Europe, and NATO has to respond," the general said.
Bradshaw was the No. 2 allied commander in Afghanistan prior to taking his post at NATO in March.