WASHINGTON — When President Barack Obama and other world leaders gather in Germany next week, Russia’s Vladimir Putin will be left off the guest list, part of his punishment for more than a year of alleged Kremlin-supported aggression in Ukraine.

But despite vows from Obama and his European counterparts to isolate Putin as long as the crisis in Ukraine remains unresolved, the Russian leader is still a central player in major international affairs, including the U.S.-led nuclear talks with Iran.

Just this month, German Chancellor Angela Merkel traveled to Moscow for talks with Putin and Secretary of State John Kerry went to Sochi to confer with the Russian leader. Putin and British Prime Minister David Cameron also spoke by phone in recent days and agreed to resume talks aimed at ending Syria’s civil war, another matter where Putin’s cooperation is crucial.

U.S. officials defend the engagement with Russia, saying it’s limited to areas where Moscow and the West have shared interests. Outreach to Putin on such matters, officials argue, should not be seen as a sign that the West has accepted the status quo in Ukraine, where pro-Russian separatists continue to stoke instability.

“It makes sense to cooperate where there is a clear mutual interest as long as you’re not being asked to back off matters of principle that matter to the security and well-being of your country and your allies and your friends,” Vice President Joe Biden said Wednesday.