EU and US consider 'deeper sanctions' against Russia
The US and EU are discussing "deeper sanctions" against Russia if there are "further incursions into Ukraine".
US President Barack Obama said "energy is obviously a central focus of our efforts", acknowledging it "will have some impact on the global economy".
He was speaking after talks in Brussels with EU leaders.
In a keynote speech later, he said Russians "will recognise that they cannot achieve security, prosperity and status... through brute force".
"That's why, throughout this crisis, we will combine our substantial pressure on Russia with an open door for diplomacy," he told an audience at the end of his visit to Brussels.
Tensions are high between the West and Russia after Ukraine's southern peninsula of Crimea was annexed by Russian forces earlier this month after a referendum which Kiev and its Western allies considered illegal.
Mr Obama, along with EU Council President Herman Van Rompuy and EU Commission President Jose Manuel Barosso, stressed EU and US unity on the issue of Ukraine.
"The world is safer and more just when Europe and America stand as one," Mr Obama said. Mr Van Rompuy called it a "crucial" relationship.
Mr Obama praised the 28-nation EU bloc for the steps it had already taken - along with the US - to penalise Russia. These have included visa bans and asset freezes against a number of Russian officials.
He said those actions were taken after Russian forces moved in to annex Crimea, and they now must consider "the potential for additional, deeper sanctions" should Moscow attempt to do the same in other parts of Ukraine.
"We recognise that in order for Russia to feel the impact of these sanctions, it will have some impact on the global economy as well as on all the countries represented here today," Mr Obama said.
Acknowledging that some EU countries were more dependent than others on Russia for energy, he said "this entire event has pointed to the need for Europe to look at how it can further diversify its energy sources".
And he said Nato must remain a "regular presence" in those eastern European countries which feel vulnerable to possible Russian intervention. He also voiced concern at the falling defence budgets of some countries.
Mr Van Rompuy called Russia's actions in Crimea "a disgrace in the 21st Century, and we will not recognise it".
Pro-Russian forces began moving it to take key sites in Crimea days after Ukraine's pro-Russian President Viktor Yanukovych was ousted. He left power following months of bloody protests over his decision to seek greater ties with Moscow rather than the EU.
This is Mr Obama's first official visit to the EU headquarters in Brussels.
He began his trip to Belgium with a visit to a cemetery in Flanders, where US soldiers killed in World War One are buried.
He paid tribute to fallen US soldiers at the American Cemetery and Memorial in Waregem, to mark 100 years since the start of WW1. Belgian King Philippe and Prime Minister Elio di Rupo were also in attendance.
Mr Obama's talks with Mr Van Rompuy and Mr Barroso also covered plans to finalise a transatlantic trade partnership, as well as efforts to tackle Iran's nuclear programme and Syria's chemical weapons.
He then met Nato Secretary-General Anders Fogh Rasmussen before delivering his keynote speech in Brussels' Palace of Fine Art.
The president flew in on Tuesday evening from the Dutch city of The Hague where he attended a summit on nuclear security with other world leaders.
Security is high in the Belgian capital, with police cordoning off areas near the EU headquarters and Mr Obama's hotel.
Some extra 800 police officers have been deployed on Brussels' streets for the duration of Mr Obama's visit.
In total, Belgium has spent 10m euros (£8.35m) on increased security.
The US president will stay in Brussels for less than 24 hours before travelling to Rome for a meeting with Pope Francis.