Envoys from the six global powers that have spent 18 months trying to negotiate a nuclear deal with Iran hope to complete an outline as early as Sunday, two days before the deadline.
British Foreign Secretary Philip Hammond said Friday diplomats hope to reach a preliminary deal "over the next 48 hours" but will keep working if they don't.
Josh Earnest, the White House press secretary, said it was "possible" the negotiators would need until early next week to complete the framework.
Teams led by Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have faced off across a table several times since Thursday morning. Hammond is expected to arrive Saturday.