The Ukraine crisis is likely to dominate trade talks between the European Union and the US, as President Barack Obama makes his first visit to Brussels.
Speaking ahead of Wednesday's summit, the president of the European Council, Herman van Rompuy, described cooperation between the EU and the US as of "critical importance".
Van Rompuy said: "At a moment when the principles of international security and diplomatic engagement are challenged in Europe and the world, the cooperation between the European Union and the United States is of critical importance."
Obama is also due to meet President of the European Commission José Manuel Barroso in the first EU-US summit to have taken place in more than two years.
The meeting is the third high-level diplomatic effort this week to directly or indirectly address the Ukraine crisis.
On Tuesday, following a nuclear security summit, Obama told reporters that Russia was a regional power threatening its neighbours out of weakness.
On Monday, the Group of Seven industrialised countries warned Russia it faced damaging economic sanctions if President Vladimir Putin took further action to destabilise Ukraine following the seizure of Crimea.