"The strange inspection arrangement in the agreement will allow the Iranians to lie and cheat."
"We are working hard, but a deal tonight is simply logistically impossible."
Diplomats have until Monday to reach an agreement, their third extension in two weeks.
"We're here because we believe we are making real progress," Kerry said.
"We're frankly more concerned about the quality of the deal than we are about the clock," the spokeswoman for the U.S. delegation said.
Six major powers and Iran have decided to extend the original June 30 deadline to July 7 to allow more time for negotiations.
Diplomats said the talks in Vienna would run for as long as necessary to reach a deal.
The State Department accused Tehran of kidnapping and amputations on Thursday.
Alongside monitoring trade, the financial sector has a key role to play—if its rules are set right.
The Camp David summit is just the start in a long process needed to calm a troubled region.
Only two heads of state will attend as Gulf states seek reassurance over US ties with Iran.
Tehran's top nuclear negotiator hopes diplomacy resuming this week will yield results before a self-imposed deadline for a final deal to curtail Iran's atomic program expires at the end of June.
At a high-stakes meeting at Camp David this week, the president will meet with leaders of the Gulf Arab states.
Despite initial grumbling, all four possible Republican presidential contenders in the Senate lined up behind the bill.
Giving Iran a missile defense system represents a fundamental shift in military power in the Middle East.
The Navy said it "has increased its presence in this area as a result of the current instability in Yemen."
President may take business delegation if nuclear deal clinched in first Tehran visit by western leader for years.
Ayatollah Ali Khamenei has broken his silence on the nuclear agreement.
"I neither support nor oppose the deal. Everything is in the details, it may be that the deceptive other side wants to restrict us in the details," Khamenei said in a speech broadcast live on state television.
The president tried to clarify his approach to Iran and Israel, but ended up muddying the water.