WASHINGTON - US President Barack Obama on Friday signed an executive order, imposing a ban on new investment in Crimea and barring export or import of goods, technology or services with the Ukrainian region annexed by Russia.

The president noted in his order that the move was taken to " address the Russian occupation of the Crimea region of Ukraine" and that he authorized sanctions on individuals and entities operating in Crimea by freezing their assets under US jurisdiction and denying them entry to the United States.

In another development, the Department of Treasury on Friday hit with sanctions what it called 24 Ukrainian and Russian separatist leaders and the militias or entities they lead or support.

Obama acted after the European Union took similar actions a day earlier. He said in a statement that his 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."

Moscow took over Crimea in March as an overwhelming majority of the residents on the peninsula voted to secede from Ukraine and join Russia in a crisis sparked by the ouster in February of Ukraine's pro-Russia President Viktor Yanukovych.

Washington and its European allies do not recognize the Russian annexation of Crimea, and have slapped rounds of sanctions on Moscow over the episode and what they call Russia's unending efforts to fund and arm the separatists in eastern Ukraine who have been fighting the government troops over the past months.

In his statement, Obama once again urged Russia to end its " occupation and attempted annexation" of Crimea, stop support to separatists in eastern Ukraine, and implement peace deals signed in September in the Belarusian capital city of Minsk, a refrain he echoed on Thursday when he signed into law a bill aiming for tougher sanctions on Moscow.

The so-called Ukraine Freedom Support Act authorizes Obama to apply new sanctions against Russian defense and energy firms to punish Moscow's continuing involvement in the insurgency in eastern Ukraine, and allows Obama to provide military assistance worth 350 million dollars to Ukraine, with anti-tank and anti- armor weapons on the list.

The president said, however, his administration had no intention to impose more penalties against Moscow at the moment.

Obama said he did not order fresh sanctions on Crimea under the new law, noting Washington would "review and calibrate" the sanctions with its allies and partners.

President Vladimir Putin reiterated on Thursday that Russia would not bow to external political pressure and would stay firm to protect the national interests.

Speaking at a year-end press conference, Putin said Russia was willing to mediate political talks over the Ukraine crisis.