Taking a direct shot at Russian President Vladimir Putin, President Obama told the leader Monday that the U.S. would impose additional sanctions on Kremlin officials if they don't stand down in Ukraine.

“If Russia continues to intervene in Ukraine, we stand ready to impose further sanctions,” Obama warned, shortly after his administration issued economic punishments against those close to Putin.

“The future of Ukraine must be decided by the people of Ukraine,” Obama said in brief remarks from the White House.

The U.S. earlier Monday froze the assets of seven Russian government officials and four separatist leaders, including the self-appointed prime minister of Crimea and the former prime minister of Ukraine.

The president sought to paint Russia into a corner, saying Putin would feel the effects of his go-it-alone approach to Ukraine.

"Further provocations will do nothing except to further isolate Russia and diminish its place in the world," Obama said from the briefing room.

"We can calibrate our response on whether Russia chooses to escalate or de-escalate the situation," he added.

Senior administration officials framed the sanctions as the most extensive actions taken by the U.S. against Russia since the end of the Cold War.

The measures freeze any U.S. assets held by those individuals and prevents American citizens from conducting business with them.

The administration’s move to expand sanctions on Monday was also matched by the European Union, which similarly enacted measures against 21 top officials.

While Putin was not one of the Russian officials targeted, administration officials insisted that the sanctions would “hit close to home” for the leader.

However, analysts have questioned whether the sanctions will do anything to deter Putin, who sees Ukraine as an extension of Russian power and influence.

Obama’s statement came one day after the Russian-majority region of Crimea voted overwhelmingly in a referendum to secede from Ukraine and join Russia.

The president told Putin in a phone call Sunday that the international community would not accept the legitimacy of the vote, which he said violated both Ukraine’s constitution and international law.

The international community has been unable to convince Putin to stand down in Crimea which Russian military forces seized after Moscow-backed former Ukrainian President Viktor Yanukovych was deposed from power.

Moscow claims that they are protecting the safety of the region’s Russian-ethnic population, but the U.S. and allies have accused Putin of a power grab.

Obama has urged Putin to draw back military forces in Crimea, allow international observers and resolve the standoff diplomatically with the government in Kiev.

This story was published at 11:01 a.m. and has been updated.