The pact to be announced by US President Joe Biden and the president of the EU's executive, Ursula von der Leyen, follows a day of three summits in Brussels where leaders lambasted Russia's invasion of Ukraine and offered fresh support to Kyiv.
"The single most important thing is for us to stay unified and the world continue to focus on what a brute this guy is and all the innocent people's lives that are being lost and ruined," Biden said, referring to Russian President Vladimir Putin, after attending the first day of an EU summit on Thursday.
"We have to stay fully, totally, thoroughly united."
The invasion by Europe's top gas supplier has pushed already-high energy prices to records and prompted the EU to pledge to cut Russian gas use by two-thirds this year by hiking imports from other countries and boosting renewable energy.
Biden promised the United States would deliver at least 15 billion cubic metres (bcm) more LNG to Europe this year than planned before, sources familiar with the matter said.
EU leaders will discuss on Friday what more they can to do rein in high energy bills.
Spain, Greece and others will make their case for power price caps and market intervention, while a group that includes Germany and Netherlands will push back and seek to delay such moves, diplomat said.
The divisive issue of whether to impose an embargo on Russian energy, on top of the slew of sanctions already targeted at Moscow, will also come up but no decision is expected.
Russia supplies 40 per cent of the EU's gas needs and more than a quarter of its oil imports. Those most dependent on this supply - in particular Germany - are reluctant to take a step that would have a major economic impact.
The 27 leaders will also commit to start jointly buying gas and fill storage ahead of next winter to build a buffer against further supply shocks.
