Calling Russia's actions in Crimea a "wake-up call," NATO Secretary General Anders Fogh Rasmussen told a crowd at a Washington think tank on Wednesday that Russia might not stop with Crimea.
"I see Crimea as an element in a greater pattern, in a more long-term Russian - or at least Putin strategy," he said. "So of course, our major concern now is whether he will go beyond Crimea, whether the Russia will intervene in the eastern parts."
Rasmussen said Russia's military intervention is the "gravest threat" to Europe since the Cold War.
"We had thought that such behavior had been confined to history, but it's back and it's dangerous because it violates international norms of accepted behavior," he said.
Rasmussen said that all cooperation between NATO and Russia is now under review, including suspending plans to escort Russian ships that are ferrying chemical weapons from Syria, and further sanctions could be applied.
He said that the door was not closed in finding a political solution to continue to work with Russia, but that the Kremlin's behavior must stop.
"Should Russia be considered a partner or an adversary? I have to ask that question, and many allies asked that question," he said. "So that's why we can't continue business as usual."
Beyond the size of Russia's military intervention and the fact that these actions directly border NATO countries, Rasmussen said Russia refusing to honor its commitments is especially troubling.
"Russia was among those who committed in 1994 to respect Ukraine's territorial integrity and sovereignty," he said. "Russia pledged not to threaten or use force against Ukraine. By turning its back on that agreement, Russia has called into question its credibility and reliability as an international actor."
The secretary general, who is in Washington to meet with Secretaries Kerry and Hagel as well as National Security Adviser Susan Rice, said he expects the fate of the Ukraine to be the primary issue of discussion at next month's NATO ministerial meeting.