European Council President Donald Tusk says the European Union needs a strategy for dealing with Russia that lasts for years, not months.
"The biggest challenge today is the Russian approach, not only to Ukraine but also to the EU," said Tusk, a former Polish prime minister.
Tusk was speaking at a news conference early on December 19 at the end of a summit of EU leaders in Brussels.
He called for a strategy that is "tough and responsible" for dealing with Russia and its president, Vladimir Putin, and resolving the Ukraine crisis.
Tusk said "Russia is our strategic problem, not Ukraine."
Earlier, EU leaders adopted new sanctions as further punishment for Russia's annexation of Crimea from Ukraine in March.
The new measures ban all investment in Crimea and stop European cruise ships from traveling there.
Russia's Foreign Ministry said the new sanctions slapped on Crimea are "absolutely unacceptable."
However, EU leaders said they were ready to ease sanctions if Putin was deemed to be implementing a peace plan reached with Ukraine in Minsk in September.
British Prime Minister David Cameron said "the door is always open if Russia changes its behavior."
"If it takes Russian troops out of Ukraine, and it obeys all the strictures of the Minsk agreement, these sanctions can go," Cameron told reporters after the EU summit ended, a day earlier than originally scheduled.
But German Chancellor Angela Merkel stressed: "Sanctions... can only be lifted if the reasons for them change."
Earlier, EU foreign policy chief Federica Mogherini said she derived no satisfaction from the economic woes of Russia, partially the result of sanctions action by the EU and United States.
But Mogherini said Putin and other senior Russian officials needed "a radical change in the attitude toward the rest of the world and to switch to a cooperative mode."
Washington also added sanction pressure to Putin on December 18.
President Barack Obama signed into law a bill authorizing new sanctions against Russia, but added he would not impose them "at this time."
The Ukraine Freedom Support Act authorizes -- but does not technically require -- defensive lethal aid for Kyiv and sanctions against Russian defense and energy sectors.
Obama said in a statement that the signing of the legislation "does not signal a change in the administration's sanctions policy."
Sanctions, along with plummeting oil prices, have contributed to Russia's economic turmoil, as well as the fall of its currency.
The bill passed the divided U.S. Congress unanimously.
The bill also authorizes more loan guarantees for Ukraine and additional funds to address the country's energy shortage, for U.S. international broadcasting, and for Russian democracy and civil society organizations.
In his annual news conference ahead of the holiday season, Putin blamed Kyiv for the conflict and said Russia was merely "defending" its interests.
Ukraine and the West accuse Putin of sending arms and troops to help the pro-Russian rebels in eastern Ukraine, an allegation the Kremlin denies.
Putin said Kyiv was conducting a "punitive operation" in eastern Ukraine, where more than 4,700 people have been killed since April.
With reporting by Reuters and AP