President Barack Obama froze the US assets of seven Russian officials, including top advisers to President Vladimir Putin, for supporting Crimea's vote to secede from Ukraine.

Mr Obama said that he would "increase the cost" to Russia and warned that more people could face financial punishment.

"If Russia continues to interfere in Ukraine, we stand ready to impose further sanctions," he said.

However, he added that he still believed there could be a diplomatic resolution to the crisis and that sanctions could be calibrated depending on whether Russia escalated or diminished its involvement.

The US Treasury Department has also imposed sanctions on four Ukrainians, including former president Viktor Yanukovych and others who have supported Crimea's separation, under existing authority from a previous order by Mr Obama.

Senior administration officials said that they were developing evidence against individuals in the arms industry and those they described as "Russian government cronies" to target their assets.

The administration officials explained that President Putin was not affected despite his support for the Crimean referendum because the US did not usually begin sanctions with heads of state.

President Obama's comments followed an announcement by the European Union giving details of travel bans and asset freezes on 21 people it has linked to events in Crimea.