The European Union and the US have unveiled sanctions against senior Russian officials involved in the Crimea stand-off - and warned of more to come if tensions do not ease.
Those targeted with asset freezes and travel bans include close aides of President Vladimir Putin, as well as deputy prime minister Dmitry Rogozin.
Pro-Ukrainian activists demonstrating outside the EU Council in Brussels on Monday
The measures were announced after a controversial referendum in Crimea returned a 96% vote in favour of joining Russia, with the Ukrainian parliament responding by mobilising thousands of troops.
Barack Obama announced that financial sanctions were being imposed on seven top Russian officials and four Crimea-based separatist leaders seen as undermining "democratic processes and institutions in Ukraine".
They include Vladislav Surkov and Sergey Glazyev, aides to Mr Putin, Mr Dmitry Rogozin, and the proclaimed leader of Crimea Sergei Aksyonov.
Mr Obama said America was "ready to impose further sanctions" depending on whether Russia escalated or de-escalated the situation in Ukraine.
If Moscow continued to intervene in Ukraine, he warned, it would "achieve nothing except to further isolate Russia and diminish its place in the world".
The EU is expected to publish a list of 21 individuals being hit with its asset freezes and travel bans later, after agreeing the names at a meeting of foreign ministers in Brussels earlier.
Mr Hague said: "These are individuals not just in Crimea but in Russia as well, including in the armed forces and in the parliament - people who are associated with the decisions that Russia has made about Crimea.
"I think that's an important statement of the unity and resolve on this question, coinciding of course with the measures taken by the US.
"It is possible to add to these measures of course. This is not a list that is set in stone for the future, it is possible to add other figures in future depending on how Russia reacts to the referendum in Crimea.
He also pointed to the longer-term implications for Moscow. "We have started today discussing the longer term, the need to reduce European dependence on Russian energy over many years to come," he said.
"It is some of these sorts of things that will be the biggest costs in the long term to Russia if we make no diplomatic progress over the coming weeks."
Earlier, Mr Hague suggested relations with Russia could return to Cold War levels if Mr Putin took further action in Ukraine.
"I wouldn't describe it as a new Cold War, but of course this will depend on the course of events over the coming days," he told BBC Radio 4's Today programme.
However, the sanctions were immediately mocked by Mr Rogozin, who posted on Twitter: "Comrade @BarackObama, what should do those who have neither accounts nor property abroad? Or U didn't think about it?"
Ukrainian foreign minister Andrii Deshchytsya said the government was "prepared to defend our homeland" if Russian troops advance.
He told BBC Radio 4's The World At One programme: "We are very much concerned with the deployment of Russian troops on the eastern border of Ukraine and the number of provocations made by Russians in eastern regions of Ukraine.
"We are prepared to defend our homeland if Russia will decide to move further into the eastern parts of Ukraine."
Mr Deshchytsya said the sanctions agreed today were a "step forward in mobilising the international community".
"But I think we also need to deepen our relations with the European Union," he said.