Hours before the deadline for a decision, international delegations are still working on a nuclear agreement with Iran.
Negotiators from Tehran are meeting representatives of six world powers in Vienna.
The US Congress wants the ink dry on an deal by 6am Friday morning European time.
Speaking in Vienna, US Secretary of State John Kerry said real progress is being made but the work is technical and the stakes are high.
The deal must withstand the test of time, he said, and we are talking decades. Some issues remain unresolved.
The White House says the talks can continue as long as there is genuine commitment from Iran and the world powers to resolve the issues.