G7 leaders have opened their annual summit in southern Germany by taking an uncompromising stance against Russia's actions in Ukraine.
President Barack Obama and Chancellor Angela Merkel held talks as the G7 summit of economic powers began in southern Germany.
Moscow is the target of European Union and US sanctions over its role in support of Ukrainian rebels.
Russia has been excluded from what was previously known as the G8 since the annexation of Crimea last year.
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.
As he arrived in the Bavarian Alps, Mr Obama said G7 leaders would discuss "standing up to Russian aggression" in Ukraine.
The White House issued a statement after Mr Obama's talks with Mrs Merkel, saying: "The duration of sanctions should be clearly linked to Russia's full implementation of the Minsk agreements and respect for Ukraine's sovereignty."
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.
Last September's Minsk accord, involving Russia, pro-Russian rebels and the Ukrainian government, included the establishment of a 30km buffer zone between the two sides.
But fighting has intensified in recent weeks. In the latest incident, two Ukrainian coastguards were injured when a blast ripped through their patrol boat in the port of Mariupol, though the exact circumstances remain unclear.
The European Union's President of the Council of Ministers, Donald Tusk, signalled a toughening of sanctions in a statement at the G7.
"If anyone wants to start a debate about changing the sanctions regime, the discussion could only be about strengthening it."
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.