David Cameron and Vladimir Putin will hold talks on the Syrian civil war in London on Thursday - with the Russian President in town to watch Olympic judo.
The British Prime Minister and Russian President will discuss the crisis during a bilateral summit in Downing Street.
Cameron is likely to raise Russia's continual blocking of a United Nations Security Council resolution designed to increase the pressure on President Bashar Assad as he presses on with his brutal crackdown on pro-democracy fighters in a conflict which has led to at least 19,000 deaths.
The high-level talks at No 10 come two weeks after Russia and China vetoed the latest UN attempt to end the bloodshed, and as Syrian government forces continue their battle to regain full control of Syria's biggest city, Aleppo.
Russia's backing for President Assad has meant repeated efforts for a new UN resolution have failed.
UN relief co-ordinator Baroness Amos claimed today that the lack of a resolution to establish a safe zone in Syria was hampering attempts to help civilians caught up in fighting in the Middle Eastern state.
Lady Amos said a resolution would be needed to establish a secure corridor for the delivery of aid supplies or a safe zone for people fleeing violence in the 17-month uprising.
Describing the conditions in Aleppo and other parts of Syria as "extremely worrying", the UN's under-secretary-general for humanitarian affairs said the overriding need was for an effective ceasefire.
But Assad hailed his army's military action against what he termed "terrorist criminal gangs", saying their battle will determine the fate of the country.
Lady Amos told the BBC Radio 4 Today programme: "It is extremely worrying, and it is not just Aleppo. A couple of nights ago we had renewed conflict in parts of Damascus too."
Many people in Aleppo are sheltering in schools, and around 7,000 are believed to be gathered in the city's university, she said.
There was also concern that people trying to flee to neighbouring countries are being stopped at roadblocks or targeted by snipers.
"What we need first of all is a ceasefire, which we have not been able to get, so people can be safe," said Lady Amos.
"Without that, some kind of corridor or safe zone of course would be something we could look at if we could make it secure.
"The difficulty is we have no Security Council resolution which would enable that to happen. You don't just get the government forces and the opposition forces to agree - you need to be able to police and secure that corridor or safe zone in some way so that people feel safe to go there and so someone doesn't break that agreement.
"At the moment it is very difficult to see how we can do that."
Meanwhile, Cameron was also under pressure to raise the 2006 death of Kremlin critic Alexander Litvinenko, who was allegedly poisoned with Polonium 210 by a Russian agent, with President Putin.
Litvinenko's widow, Marina, condemned the Russian leader's London visit, saying she was "dismayed".
She added: "Putin's agents killed my husband, a British citizen, in the centre of the British capital and contaminated hundreds of innocent people with radioactive poison.
"To show that Putin is not welcome here, I urge my fellow Londoners to put on a white ribbon, the symbol of Russian protest, on the day Mr Putin arrives."
Judo black belt President Putin will reportedly use his visit to the city, his first trip to the capital for nine years, to watch the martial art, his favourite sport, at the Olympic Games.