In an interview with US networks recorded ahead of a meeting with US President Barack Obama, Putin said Syrian President Bashar al-Assad deserved international support as he was fighting terrorist organizations, Reuters reported.
Obama and Putin are scheduled to talk on Monday after Putin addresses the United Nations, although White House and Kremlin officials have disagreed on what the two leaders will discuss and even who initiated the meeting.
"In my opinion, provision of military support to illegal structures runs counter to the principles of modern international law and the United Nations Charter," he said in an excerpt of an interview with US television networks CBS and PBS released by the Kremlin.
Russia's sudden military build-up this month in support of Assad and a refugee crisis that has spilled over from the region into Europe have lent new urgency to attempts to resolve the Syria conflict.
The new US tack on Syria could bring together Russia, Saudi Arabia and countries such as Turkey and Qatar, which support militant groups in Syria.
US-Russian relations have slumped to a post-Cold War low over the Ukraine crisis, though the two sides shares concerns about the threat posed by ISIL, while disagreeing on the approach.
Putin says Damascus should be included in international efforts to fight ISIL, a demand the United States rejects, and he criticized US plans to train up to 5,400 Syrian militants to fight the terrorist group.
"It turns out that only 60 of these fighters have been properly trained, and as few as four or five people actually carry weapons," he said. "The rest of them have deserted with the American weapons to join ISIL," he said .
Putin said Russia's support for the Assad government was based on the UN Charter.
"We have been providing assistance to legitimate government entities only," he said. "As of today it has taken the form of weapons supplies to the Syrian government, personnel training and humanitarian aid to the Syrian people."