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.