Barack Obama has pressed David Cameron to maintain Britain's commitment to meeting the Nato target of spending 2% of GDP on defence.
Mr Cameron has not committed to the 2% benchmark beyond March 2016.
Any decision must "await Chancellor George Osborne's Spending Review in the autumn", he says.
In talks at the G7 summit in Germany, the US President also said he was "looking forward" to Britain remaining in the European Union.
Mr Osborne's demand last week for a further £500m in Ministry of Defence cuts has fuelled concerns that the UK may drop below the 2% level next year.
Following the talks between the two men at the Schloss Elmau castle in the Bavarian Alps, a Downing Street source said Mr Obama had "touched on" the issue of whether Britain would continue to meet the 2% target.
"The president underlined the importance of the UK and US as the two pillars of Nato, and said he accepted the fiscal challenge but hoped that the UK would find a way to meet it," said the source.
Asked if Mr Cameron had offered any assurances in response, the source declined to "give a running commentary", but said the PM had pointed to the numerous military operations around the world to which Britain has contributed.
Before the talks, Mr Cameron told reporters: "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."
Touching on the subject of Britain staying in the EU, Mr Obama said: "I would note that one of the great values of having the United Kingdom in the European Union is its leadership and strength on a whole host of global challenges.
"And so we very much are looking forward to the United Kingdom staying a part of the European Union because we think its influence is positive not just for Europe but also for the world."
Mr Cameron wants to renegotiate the UK's EU membership ahead of an in/out referendum by 2017.
He has opened talks with other European leaders including German Chancellor Angela Merkel and Polish Prime Minister Ewa Kopacz - who both want to ensure the UK stays in the EU.
Public to decide
Speaking at the G7 summit in Germany, the PM said the British public, not "individual parties", would decide whether the UK stays in the EU.
This comes after it emerged that some 50 Eurosceptic MPs have formed a group called Conservatives for Britain that wants Westminster to be sovereign over European Union law.
Before meeting Mr Obama for one-on-one talks at the G7 summit in Germany, the prime minister announced the deployment of a further 125 army trainers to Iraq.
The trainers will help the Baghdad authorities take on the Islamic State extremist group which has seized large swathes of the country as well as parts of neighbouring Syria.