Russian President Vladimir Putin told CBS's "60 Minutes" on Sunday that U.S. support of so-called moderate rebel forces in Syria is illegal.

"In my opinion, provision of military support to illegal structures runs counter to the principles of modern international law and the United Nations Charter," Putin told CBS's Charlie Rose, according to Reuters.

The U.S. has been working to overthrow the Syrian government since it started backing peaceful protesters in 2007. More recently, the CIA and Pentagon have been providing arms, training and funding to various rebel groups. One of these programs, which Congress approved with a $500 million price tag, has been ineffective, Putin said.

"It turns out that only 60 of these fighters have been properly trained, and as few as four or five people actually carry weapons," Putin said. "The rest of them have deserted with the American weapons to join" the Islamic State group.

The Obama administration admitted on Friday that another group of U.S.-armed rebels had indeed voluntarily handed over at least 25 percent of their weapons and vehicles to al-Qaeda affiliates, reported The Hill.

Putin is scheduled to meet with President Obama at the United Nations General Assembly on Monday, where the two are expected to discuss how to move forward in Syria and Ukraine.

Russia recently began sending weapons to the Syrian government to prop up President Bashar al-Assad, who is fighting against both the Islamic State group and separate U.S.-backed rebels. Some say Russia's involvement has forced the U.S. to consider more diplomatic approaches to dealing with Assad.

"We act based on the United Nations Charter, i.e., the fundamental principles of modern international law, according to which this or that type of aid, including military assistance, can and must be provided exclusively to the legitimate government of one country or another, upon its consent or request, or upon the decision of the United Nations Security Council," Putin said, reported RT.

Assad is the only legitimate force fighting against terrorism in the region and therefore must be supported, according to Putin, who said he would like to see a unified effort between the U.S., Russia and Syria.

Obama said Monday at the U.N. that the "United States is willing to work with any nation, including Russia and Iran, but we must agree that after so much carnage there cannot be a return to the previous status quo," according to USA Today. "When a dictator slaughters tens of thousands of his own people that's not just a matter of one country's internal affairs. There is no room for accommodating an apocalyptic cult like" the Islamic State group.