David Cameron has called on Vladimir Putin to "change direction" and turn Russia's fire against the Islamic State (IS) terror group in Syria, rather than seeking to shore up the "butcher" President Bashar Assad with attacks on the moderate opposition to his regime.
The Prime Minister's call came as he announced plans to bolster the UK military's ability to tackle IS, which he warned was "plotting murder and mayhem on the streets of Britain".
The RAF's fleet of Reaper drones will be more than doubled - to over 20 - and upgraded with the latest "Protector" technology to increase its ability to identify, track, deter and counter potential threats.
Special forces troops, including the SAS, will be equipped with new specialist weapons and clothing.
Last Wednesday, Russia launched a campaign of airstrikes which the Kremlin says are aimed at IS - also known as Isil, Isis or Daesh - which has seized large swathes of Syria and neighbouring Iraq.
But Mr Cameron told BBC1's Andrew Marr Show the majority of Russian bombs have so far fallen in areas of the country controlled by more moderate forces.
He said: "Tragically, what has happened is that most of the Russian airstrikes, as far as we have been able to see so far, have been in parts of Syria not controlled by Isil but by other opponents of the regime.
"They are backing the butcher Assad, which is a terrible mistake for them and for the world. It is going to make the region more unstable, it will lead to further radicalisation and increase terrorism.
"I would say to them: 'Change direction, join us in attacking Isil, but recognise that if we want to have a secure region, we need an alternative leader to Assad'.
"He can't unite the Syrian people. Assad has driven far more people from their homes with the barrel bombs and the brutality than even the brutal death cult of Isil."
Speaking as the Conservative annual conference opened in Manchester, Mr Cameron said that "our national security will be greatly assisted by degrading and ultimately destroying Isil both in Iraq in Syria".
But he restated his position he will not order RAF airstrikes against Isil to be extended from Iraq into Syria until he has secured the approval of the House of Commons.
"Obviously, we are not going to do that until we have a vote in the House of Commons," he said.
"But meanwhile we are supporting the Americans and others who are attacking Isil in Syria. We have used drones in Syria where there is a national interest at stake and we will do so again.
"Isil - this appalling death cult - are people who are plotting murder and mayhem on the streets of Britain, and we have got to have the capabilities to deal with them, which is why we have announced today we are going to be increasing the fleet of drones we have."