President Barack Obama has signed an executive order banning the export of goods, technology and services to Crimea.
The executive order also imposes new sanctions on certain Russian and Ukrainian individuals and companies.
The West has imposed various sanctions on Russia since it annexed Crimea after the removal of Ukraine’s pro-Moscow President Viktor Yanukovych in February.
Barack Obama said his latest decision was to show that the US would not accept Russia’s annexation.
The annexation was followed in April by pro-Russian separatists taking control of parts of the Donetsk and Luhansk regions of eastern Ukraine and declaring independence.
Some 4,700 people have died and another million have been displaced by fighting since then, the UN says.
On December 19, five Ukrainian soldiers were killed in fighting – the highest death toll since the latest attempt at a ceasefire began on 9 December.
Barack Obama said in a statement: “The executive order is intended to provide clarity to US corporations doing business in the region and reaffirm that the United States will not accept Russia’s occupation and attempted annexation of Crimea.”
In addition to the goods, technology and services ban, US individuals or companies cannot now buy any real estate or businesses in Crimea or fund Crimean firms.
The new measures also include sanctions on 24 Ukrainian and Russian individuals and on a number of companies deemed to be destabilizing Ukraine.
They include the Russian equity investment group, Marshall Capital Partners, and the Night Wolves biker group, over its involvement in Crimean military action.
Meanwhile, the EU imposed its own new sanctions against the Crimea region.
Like the EU, Barack Obama said he would not yet impose new sanctions on Russia, urging it again to de-escalate the tension in eastern Ukraine.
Russian Foreign Minister Sergei Lavrov said that “threatening new sanctions against Russia could undermine the possibility of normal cooperation between our countries for a long time”.
Barack Obama said: “I again call on Russia to end its occupation and attempted annexation of Crimea, cease its support to separatists in eastern Ukraine, and fulfill its commitments under the Minsk agreement.”
The agreement signed by Ukraine and the rebels in Minsk, in Belarus, in September, put in place a ceasefire and set out the terms for a peace process.
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