NATO members will be “judged harshly” if they do not increase defense spending in response to the challenge now posed by Russia, Defense Secretary Chuck Hagel told U.S. allies Friday.

“Talking amongst ourselves is no longer good enough. . . . Leaders across our governments must understand the consequences” of the current fiscal impasse, Hagel said in an address at the Wilson Center.

Hagel’s speech is the toughest yet among a series of Obama administration appeals to the alliance to increase defense spending that has been lagging for years. Only a handful of NATO’s 28 members meet the agreed budget target of 2 percent of their gross domestic product.

In March, President Obama appealed to “every NATO member state . . . [to] step up and carry its share of the burden by showing the political will to invest in our collective defense.” As Russian troops began to mass on the Ukrainian border, Obama said that only alliance strength would push the Kremlin toward a diplomatic solution.

Since then, tension in eastern Ukraine has increased exponentially. As government troops launched an offensive Friday morning to retake the city of Slovyansk, pro-Russian separatists apparently shot down two Ukrainian military helicopters. The Kremlin warned that there was no further point in trying to negotiate a diplomatic agreement.

“For decades — from the early days of the Cold War — American defense secretaries have called on European allies to ramp up their defense investment,” Hagel said. One of the biggest obstacles, he said, has been a sense that the age of aggression in Europe was over.

“Russia’s actions in Ukraine shatter that myth and usher in bracing new realities,” Hagel said.

“We must see renewed financial commitments from all NATO members,” he said.

Top NATO and U.S. officials have called for budget talks at the highest levels when the next NATO summit is held in Wales in September.

Hagel called for a meeting of senior financial ministers and budget officials even before that “to receive detailed briefings from alliance military leaders on the challenges we face.”

Hagel noted that U.S. defense secretaries have been making the same appeal to Europe for decades, particularly since the end of the Cold War convinced them that Europe was secure. “Russia’s actions in Ukraine shatter that myth and usher in bracing new realities,” he said.

NATO’s “lopsided burden” has fallen disproportionately on the United States, Hagel said. “Today, America’s GDP is smaller than the combined GDP’s of our 27 NATO allies, but America’s defense spending is three times our Allies combined defense spending,” he said.

Although Hagel did not mention it, much of U.S. spending has come about as a result of three wars — two in Iraq and one in Afghanistan — in which NATO only partially participated.