GENEVA (Reuters) - An agreement that would open the door to ending a decade-long nuclear standoff between Iran and six world powers is possible this week if negotiators exert maximum effort, Iran's foreign minister said on Thursday.

"If everyone tries their best, we may have one," Mohammad Javad Zarif told reporters after a breakfast meeting with European Union foreign policy chief Catherine Ashton.

Zarif was asked if an agreement was conceivable at the talks between Iran and the five permanent U.N. Security Council members and Germany on Thursday and Friday in Geneva.

"We expect serious negotiations. It's possible," he said.

A senior U.S. administration official said on Wednesday that Washington could offer Iran "limited, temporary, reversible" relief from economic sanctions if Tehran took steps to curtail its atomic program.

The sanctions were imposed to put pressure on Iran to stop aspects of its atomic program that the West fears are aimed at developing nuclear weapons. Iran denies any such intention, saying it wants nuclear power to generate electricity.

The administration official declined to provide details on sanctions relief. But another U.S. official told Reuters that Washington might allow the freeing-up of some of Iran's assets that have been frozen in foreign accounts. This would enable Tehran to access much-needed cash without disrupting the complicated web of U.S., EU and U.N. sanctions.

Zarif said in a Facebook post: "After the opening assembly, my colleagues and representatives from the (six) countries will begin very difficult negotiations that have now reached a stage of entering into details, which is always difficult and detailed."

While both he and the senior administration official voiced optimism for a possible deal, Washington made clear on Wednesday that the six powers and Iran still had work to do to secure an agreement acceptable to all side.

"Nothing is yet agreed to," the U.S. administration official said. "There are gaps between our two sides which remain quite real. But this (a deal) is our goal."

The sanctions over Tehran's refusal to halt uranium enrichment and other sensitive nuclear work have slashed Iran's oil revenues, fuelled inflation and caused the value of the country's currency to plummet.

(Reporting by Fredrik Dahl, Justyna Pawlak, Louis Charbonneau and Yeganeh Torbati; writing by Louis Charbonneau; Editing by Mark Trevelyan)