President Barack Obama has said G7 leaders will discuss “standing up to Russian aggression” in Ukraine, as he arrived at the summit in Germany.
Trade, violent extremism and climate change would also feature, he said.
The annual summit of leading Western economic powers is taking place in the Bavarian Alps.
Russia has been excluded from what was previously known as the G8, since the annexation of Crimea last year. It backs rebels in eastern Ukraine.
There are concerns that President Vladimir Putin is deliberately building up further military pressure in Ukraine, and Russia already faces sanctions imposed by the US and European countries.
The West accuses Russia of sending military forces into eastern Ukraine to help the rebels – a charge echoed by analysts. Moscow denies this, saying any Russian soldiers there are volunteers.
Germany, Britain and the US want an agreement to offer support to any EU member state tempted to withdraw backing for the sanctions on Moscow, which are hurting the Russian economy.
UK Prime Minister David Cameron said he was hopeful that there would be a united front to ensure that sanctions were “rolled over” despite admitting that “sanctions are having an impact on all of us”.
EU sanctions are due to expire at the end of July.
Russian President Vladimir Putin said on Saturday that Russia was not a threat and had “other things to worry about”.
He told the Italian newspaper Corriere della Sera: “Only an insane person and only in a dream can imagine that Russia would suddenly attack Nato.
UK Foreign Secretary Philip Hammond echoed concerns about Russian military pressure in an interview with the BBC’s Andrew Marr Show.
But, when asked if the US should redeploy intermediate-range nuclear missiles in Europe, Mr Hammond said the West had a “delicate act to perform”.
“We’ve got to send a clear signal to Russia that we will not allow them to transgress our red lines. At the same time we have to recognise that the Russians do have a sense of being surrounded and under attack, and we don’t want to make unnecessary provocations.”
Mr Obama was greeted in the town of Kruen by Germany’s Chancellor Merkel where he briefly outlined his priorities.
“We’re going to discuss our shared future, the global economy that creates jobs and opportunity, maintaining a strong and prosperous European Union, forging new trade partnerships across the Atlantic, standing up to Russian aggression in Ukraine, combating threats from violent extremism to climate change.”
The two leaders then sat down to a traditional Bavarian meal of sausages and beer in the sunshine.
Greece’s debt crisis and how to tackle global warming will also be on the agenda.
Ahead of the G7 gathering, thousands of protesters marched in the nearby town of Garmisch-Partenkirchen, sparking sporadic clashes with police.
Several marchers were taken to hospital with injuries, but the violence was minor compared to some previous summits.
Security is being provided by 17,000 police officers.
The summit is also being attended by Japanese Prime Minister Shinzo Abe, UK PM David Cameron, French President Francois Hollande, Canada’s PM Stephen Harper and Italian PM Matteo Renzi.
Mrs Merkel will also be hoping to use the summit to discuss her plans for radical reform of global responses to pandemics like Ebola.
She wants to streamline and re-focus the World Health Organization, widely judged to have been ill-equipped when Ebola hit, and build up an international reserve force of doctors and scientists for deployment in a future crisis.
Mr Cameron will unveil plans for a squad of “disease detectives” ready to fly anywhere to identify new infections.
On Monday, the summit is also due to discuss militant threats from groups like Islamic State and Boko Haram with the leaders of Nigeria, Tunisia and Iraq, who form part of an “outreach” group of non-G7 countries.