The White House has said it is confident of reaching a final deal over Iran’s nuclear programme by the end-of-June deadline.
A framework agreement struck yesterday will see Iran curb nuclear activities in return for relief from sanctions.
US Republicans sceptical to the deal have threatened to derail it or impose new sanctions. President Obama said he had begun efforts to win over sceptical members of Congress. The White House spokesman Eric Schultz said Mr Obama spoke to all four congressional leaders yesterday.
Republicans control both US Houses of Congress, and there is bipartisan support for a bill which would give Congress the right to review any deal before sanctions are lifted. Mr Obama has threatened to veto it.