David Cameron is hoping for a tough message on Ukraine from the leaders of the West's major powers as the G7 summit concludes in Germany.
The Prime Minister last night held talks with US president Barack Obama at which both men agreed the need to maintain harsh sanctions against Russia until Moscow shows it has fully implemented last year's Minsk peace agreement.
But the message was overshadowed by Mr Obama's expression of concern that budget cuts in the UK may see Britain drop below Nato's target of spending 2% of GDP on the military.
Despite lecturing other leaders at last year's Nato summit in Wales on the need to hit the 2% target,, Mr Cameron has steadfastly declined to commit the UK to meeting it after March 2016, saying the decision must await Chancellor George Osborne's spending review in the autumn.
A Downing Street source said that Mr Obama "touched on" the 2% issue during his meeting with Mr Cameron, adding: "The president underlined the importance of the UK and US as the two pillars of Nato, and said he accepted the fiscal challenges but hoped that the UK would find a way to meet it."
Mr Osborne's demand last week for a further £500 million in savings from the Ministry of Defence has fuelled concerns in Washington, as well as on the Tory backbenches, that the UK may drop below the totemic 2% figure next year. US defence secretary Ashton Carter last week warned that this would suggest the UK was "disengaged" and could no longer "punch above its weight" on the international scene.
Ahead of his meeting with Mr Obama, Mr Cameron was asked what he would tell the president about Britain's plans. He replied: "I'll say exactly the same as what I'm saying now, which is we've kept our 2% promise - one of the few countries to do it - and we're having a spending review in the autumn and we'll announce the results at that time."
The PM also announced the deployment of 125 army personnel to Iraq in response to a request from Prime Minister Haider al-Abadi. It brings to 275 the number of UK military personnel in Iraq, assisting government forces and Kurdish authorities in their battle against the Islamic State (IS) militants who have occupied large parts of the country and neighbouring Syria.
Most of the new troops will be involved in operations against improvised explosive devices (IEDs), which are routinely left by IS militants withdrawing from occupied areas as booby-traps on the roads and in cars, homes and official buildings.
While Britain's mission in Iraq has so far been largely focused on the Kurdish capital Irbil, the new troops will be stationed at a number of bases around the country, including Baghdad. They will remain inside bases and will not be deployed in the field.
Mr Cameron described Islamist extremist terror as "the biggest challenge" faced by the UK and the international community. Mr Abadi and Nigeria's Muhammadu Buhari are among a number of African and Middle Eastern leaders meeting the G7 leaders tomorrow to discuss the threat from groups such as IS and Boko Haram, who have taken hundreds of hostages and killed thousands in the north of Nigeria.
The summit's host, German chancellor Angela Merkel, has said she is hoping for a strong G7 statement in support of continued sanctions against Russia. EU leaders must decide at a summit in Brussels at the end of June whether to renew economic and financial measures, introduced in lockstep with similar US sanctions last year, which would otherwise lapse the following month.
With Greece's Prime Minister Alexis Tsipras due to meet Russian president Vladimir Putin on June 18 to discuss his country's debt crisis, there are concerns that Athens may break ranks and block their renewal.
Mr Obama stressed the importance of "maintaining the sanctions regime to put pressure on Russia and separatist forces to implement wholly the Minsk agreement".
He added: "We think there can be a peaceful and diplomatic resolution to the problem, but it will require that Europe and the United States and partners across the world stay vigilant and stay focused on the importance of upholding the principles of territorial integrity and sovereignty."
Russia was ejected from the G8 last year following its annexation of Crimea and interference in eastern Ukraine, where Western nations allege it has given military backing to separatist rebels - something Mr Putin denies.
Asked whether he would back financial incentives to keep on board countries such as Greece which have suffered as a result of the sanctions, Mr Cameron said: "Europe has been united on sanctions ... We need to make sure Europe remains united.
"It has an impact on all countries in terms of putting sanctions on another country. Britain hasn't let our pre-eminence in financial services get in the way of taking a robust response to Russian-backed aggression and I don't think other countries should either."
European Council president Donald Tusk told reporters: "If anyone wants to start a debate about changing the sanctions regime, the discussion could only be about strengthening them."
The summit was taking place amid tight security in the spectacular mountain-top setting of Schloss Elmau in the Bavarian Alps. Demonstrators clashed with police in the nearby town of Garmisch-Partenkirchen, while a few hundred hiked up to the venue to protest outside its perimeter fence.
Campaign group One released giant balloons featuring the faces of the seven leaders - Mr Cameron, Mrs Merkel, Mr Obama, French president Francois Hollande and prime ministers Matteo Renzi of Italy, Shinzo Abe of Japan and Stephen Harper of Canada - in a plea for them not to "talk hot air" about global warming.