Billionaire investor George Soros has vehemently condemned Russia's bombing campaign in Syria, accusing President Vladimir Putin of aggressively exploiting a power void in the White House ahead of the upcoming U.S. presidential elections.

"The world is witnessing a humanitarian catastrophe of historic proportions. It is happening in Syria. It is being perpetrated by the president of Russia, Vladimir Putin, in support of his protégé, (Syrian President) Bashar al-Assad," he said in a new statement on his website posted on Thursday morning.

"When the facts are fully established, Putin's bombing of Aleppo will be viewed as among the modern world's most egregious war crimes."

The press office of the Russian president did not immediately respond when contacted by CNBC via email.

Syria's civil war - now in its sixth year - was reignited last month after the breakdown of a cease-fire. This has increased tensions between the U.S. and Russia, which back opposite sides in Syria.

Russia has joined Syrian government forces in an escalation of airstrikes against rebel-held areas in the northern city of Aleppo. Thousands of civilians are trapped in the city and the U.K.-based Syrian Observatory of Human Rights and the United Nations have warned that the death toll is rising due to the airstrikes.

Soros is no stranger to geopolitics and has used to his Open Society Foundations (OSF) to fund a range of programs around the world. In September, Soros announced that he would invest up to $500 million in programs and companies benefiting migrants and refugees fleeing life-threatening situations.

In his statement Thursday, the investor said that Russian planes were bombing the civilian population of Aleppo to assist Syrian government forces on the ground.

"President Putin is moving aggressively to exploit the four months between now and the January 20 U.S. presidential inauguration, based on a callous political calculation," he added in the statement.

"I appeal to the people of Russia, Europe, and the rest of the world not to stand idly by, but to spread the word and voice their outrage. An outpouring of public opinion could induce President Putin to put an end to his heinous crimes against humanity."

Russia has previously banned pro-democracy charities founded by Soros , including the OSF, saying the organization posed a threat to both state security and the Russian constitution.

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