President Barack Obama warns president-elect Donald Trump to keep his distance from Russia, saying its President Vladimir Putin is a force for evil.

During a press conference in Berlin with German Chancellor Angela Merkel, Obama explained that his relationship with Russia was determined by whether or not Putin’s interests aligned with American principles.

But he express concerns that Trump would try a more practical approach, despite Russian violations of human rights, particularly with the conflict in Syria. He urged Trump to “stand up” to Russia.

“My hope is he does not simply take a real politic approach and suggest that, you know, if we just cut some deals with Russia, even if it hurts people or even if it violates international norms or even if it leaves smaller countries vulnerable, or creates long-term problems in regions like Syria, that we just do whatever’s convenient at the time,” he said.

He said that Russia and the United States have big differences when it came to principles of democracy, freedom of speech, rule of law, national sovereignty.

“On issues like Ukraine, on issues like Syria, we’ve had very significant differences,” he warned.

Obama said he welcomed Trump’s support of NATO, despite campaign rhetoric that suggested otherwise.