Western nations are 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, 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 the Prime Minister regards the dinner, at the Lough Erne golfing resort near Enniskillen, as "an opportunity for a clarifying moment on Syria".
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; and 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.
Meanwhile, 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.