Western nations were on Monday evening facing down Russian president Vladimir Putin at the G8 summit in Northern Ireland over his support for the regime of Bashar Assad in Syria.
Divisions between Moscow and the West have been laid bare at the annual gathering of world leaders, which comes days after President Barack Obama suggested the US may send weapons to the Syrian opposition.
In response Mr Putin warned that the opposition includes extremists opposed to European values, who "eat the organs" of their enemies.
David Cameron has put Syria top of the agenda for the summit dinner this evening, at which he aims to seek consensus on five key principles which he hopes could restore momentum towards a proposed peace conference in Geneva.
Downing Street said that the Prime Minister regards the dinner, at the Lough Erne golfing resort near Enniskillen, as "an opportunity for a clarifying moment on Syria".
No announcements are expected overnight on the outcome of the talks, but Mr Cameron wants a clear statement of intent in the final summit communique on Tuesday afternoon.
And a British official said the PM was ready to go ahead with a statement whether Russia signs up or not - a high-stakes diplomatic gamble which effectively presents Mr Putin with an ultimatum to engage with the process or be cast as the stumbling block to peace.
Mr Cameron has deliberately chosen five subjects for debate designed to focus minds on areas of possible consensus on the way forward, without necessarily isolating Russia.
He asked fellow leaders - including Mr Putin, Mr Obama, German chancellor Angela Merkel, French president Francois Hollande, Italian PM Enrico Letta and Japan's Shinzo Abe - to consider whether the G8 can come together behind a consistent view on:
:: Demanding access for humanitarian aid to reach Syria's people;
:: Taking on extremists on all sides of the conflict;
:: Condemning the use of chemical weapons as unacceptable;
:: Exploring whether the G8 could play a role in stabilising Syria after any change in regime;
:: Supporting a political transition to a new government executive authority in Damascus which can command the consent of the Syrian people.
Notably, Mr Putin could sign up to opposition to chemical weapons without accepting US and UK assertions that the Assad regime has used nerve gas sarin on its own people - something which the Kremlin challenges. More awkward could be an expression of support for a transitional government which is unlikely to include Assad, whose most prominent international backer Mr Putin has been.
The Russian and American presidents met on Monday for face-to-face talks which are thought to have focused on Syria.
Mr Obama acknowledged the pair had "differing perspectives" on the issue but had a shared interest in reducing the violence and preventing the use of chemical weapons.
The Russian leader said "our opinions do not coincide" but "all of us have the intention to stop the violence in Syria and to stop the growth of victims and to solve the situation peacefully".
Assad used an interview with a German newspaper to warn that if European countries agreed to arm the rebels trying to oust him "Europe's backyard becomes a terrorists' place".
Mr Cameron took the opportunity of a 30-minute ride with Mr Obama in his armoured limousine - nicknamed The Beast - to discuss his priorities for the summit, including progress on Syria as well as action to restore global economic growth.
The US and European Union launched negotiations on a comprehensive transatlantic free trade deal which the Prime Minister believes could boost the world economy by as much as £100 billion - including £10 billion in the UK alone, the equivalent of £380 for every British household.
Tomorrow he hopes to secure agreement on action to block the payment of ransoms to terrorist kidnappers, as well as an international initiative to tackle tax evasion and aggressive tax avoidance by sharing information between tax authorities.
But campaigners raised concerns that any agreement on tax information-sharing may be confined only to the rich world, leaving out the developing countries which miss out on billions in tax revenues because of individuals and companies secreting their wealth in offshore bolt-holes.
Adrian Lovett, Europe executive director at development campaign group One, said: "G8 countries need to tackle the scourge of 'phantom firms', by agreeing to disclose who really controls companies and trusts, through public registries. This will ensure that such information is available not only to tax and law enforcement authorities but also to ordinary citizens, the media and others who want to follow the money and root out corruption. The G8 must also ensure that the system they put in place for sharing tax information involves developing countries from the start.
"By taking these steps, the G8 will not only put its own house in order, but do so in ways that help the fight against extreme poverty. The UK must continue to lead the way and lobby its G8 partners, with no let-up in pace or ambition. G8 leaders must decide whether they want to shape the transparency revolution or resist the tide of history."