Obama said earlier that Moscow previously had focused on supporting Assad, but the crash of the Russian A321 plane over Egypt’s Sinai Peninsula may change its approach.
"The strengthening of the Islamic State became possible partially due to irresponsible policies of the United States. Instead of concentrating joint efforts on fighting terrorism, the United States and its allies decided to fight against the lawfully elected president of Syria Bashar Assad," Medvedev told journalists.
According to the official, Washington once contributed to strengthening of the al-Qaeda militants, which led to the September 11, 2001, terrorist attacks in the United States. These events show that only joint efforts should be used to fight terrorism, not dividing allies into "us and them," Medvedev stressed.
Syria has been in a state of civil war since 2011, with government forces countering the so-called moderate rebels and several extremist groups, including the Islamic State and the Nusra Front.
Unlike Russia, the United States and its European allies do not see President Bashar Assad as the country's legitimate leader, and support the Syrian opposition which aims to oust the president.
Russia has been launching airstrikes against the ISIL on Assad’s request since September 30. A US-led international coalition has also been conducting an aerial campaign against the militants, but without approval of Damascus or the United Nations.