U.S. President Barack Obama continued to beat the sanctions drum against Russia this week during a trip to Europe, though experts doubt such measures could force Moscow to withdraw from Crimea.

Obama is visiting Europe in a bid to get European leaders onboard with harsher sanctions on Russia after the latter's recent absorption of Crimea.

Obama spoke to European Council President Herman Van Rompuy and European Commission President Jose Manuel Barroso behind closed doors on Wednesday, and later told reporters that the issue dominated discussions.

The United States has castigated the Russian moves as contrary to international law and against Ukraine's constitution, though Russian President Vladimir Putin defended his moves citing the need to protect ethnic Russians in Crimea.

Initially, critics blasted Obama's limited sanctions as tepid and lackluster, after his administration froze assets of a handful of Russian leaders and prevented them from obtaining visas to travel to the West.

"The sanctions have been weak because the Obama administration has been looking for a graduated strategy should the future require stronger sanctions," Brookings Institution's senior fellow Darrell West told Xinhua in an interview.

"The (Obama) administration understands it is not able to force a withdrawal from Crimea, but it wants to make sure there is no further expansion and no movement into the rest of Ukraine," West said.

David Clark, chairman of the Russia Foundation, told Xinhua that if Russia takes further moves into Ukraine, it would most likely persuade the European Union to back tougher sanctions.

Further targeted measures against Russian officials involved in facilitating the absorption of Crimea are possible, as are measures to ban economic contacts and transport connections with Crimea, he said.

"There probably won't be agreement to significantly expand the sanctions regime as things stand. Europe is divided and struggled to agree to the very limited sanctions that have already been put in place," Clark said, adding that Germany is particularly hesitant.

Although it deplores Putin's actions, Germany has an influential business lobby concerned about the commercial implications of sanctions and a strong historical aversion to conflict with Russia, he said.

Putin's response to sanctions is likely to be some forms of retaliation. At the diplomatic level, this could involve the withdrawal of cooperation on issues where the West has sought Russia's help, such as Iran and Afghanistan. At the economic level, it could mean the disruption of energy supplies to Europe, he said.

Although sanctions would be far more costly to Russia than the west, "there is little chance that such measures will force a (Russian) withdrawal from Crimea," Clark added.