After basis of agreement was reached last week, cracks start to show between world leaders.
A US Senate committee is set to discuss legislation which, if passed, could prevent the White House from lifting sanctions on Iran.
The passing of such legislation could possibly endanger the nuclear talks between Western powers and Iran, that resulted in a preliminary deal earlier this month.
US President Barack Obama has threatened to veto the bill but if there is enough support, Congress could override his decision.
In order to get the two-thirds majority, the Republicans will need the support of at least 13 Democrats in the Senate and roughly 40 from the House of Representatives.