Defense Secretary Ashton Carter termed Russian President Vladimir Putin a “malign influence” in Eastern Europe and said the Russian leader’s vow to expand Moscow’s cache of nuclear missiles was “loose rhetoric.”

Amid continuing tensions over the conflict in Ukraine, the U.S. will “keep the door open” for Russia to choose better relations, Carter said, something he indicated was a long shot in remarks to reporters aboard his Pentagon jet on Sunday.

“Russia might not change under Vladimir Putin or even thereafter,” Carter said.

The Pentagon chief criticized Putin’s announcement on June 16 that Russia intends to field 40 additional intercontinental ballistic missiles able to outwit anti-missile defenses. Carter said it was “not appropriate behavior in my judgment.”

Carter spoke at the start of a week-long trip to Europe intended to highlight NATO’s efforts to adapt to a new- generation Russian threat.

The North Atlantic Treaty Organization, now in its seventh decade, is drawing up a “new playbook” to counter unconventional Russian techniques that were employed in the 2014 annexation of Ukraine’s Crimean peninsula, Carter said.

Russia’s mix of information warfare, irregular military forces and covert cross-border arms shipments, which Carter labeled “hybrid warfare,” continues to fuel a separatist rebellion in eastern Ukraine.

U.S. Senator John McCain, speaking in Kiev on Sunday, said it was in the interest of the U.S. and Europe to “reverse” Putin’s aggression. The Russian leader “may continue to pursue a strategy of just biting off small chunks of Ukraine,” said McCain, chairman of the Senate Armed Services Committee.

Saber-Rattling Slammed

The NATO alliance, which just a few years ago appeared adrift, is also confronting Islamic extremism and refugee flows from chaotic regions of North Africa, Carter said.

While slamming Putin’s saber-rattling, Carter said the U.S. still seeks Russian cooperation on issues such as the Iranian nuclear program and a settlement of the Syrian civil war, now into a fourth year. The U.S. is taking a “strong but balanced approach” to relations with its former Cold War adversary, Carter concluded.

Carter on Monday will become the first U.S. defense secretary in more than a decade to visit Berlin, where he is expected to give more details about NATO’s “new playbook” during a speech in the German capital.

The alliance is trying to beef up its ability to respond quickly to crises, especially in countries bordering Russian territory, and deal with unconventional threats like cyberattacks.

‘Short Timelines’

The U.S. and its European allies must be “able to respond on short timelines because that’s how events today unfold, unlike a quarter let alone a half a century ago,” Carter said.

Tuesday, Carter will visit a cybersecurity center in Tallinn, Estonia, and tour a U.S. warship docked there that recently participated in NATO exercises.

Later in the week he heads to Brussels for talks with NATO defense ministers. Russian intentions and the NATO training mission in Afghanistan, along with NATO modernization, are expected to be on the agenda.