The Obama administration is sending both its second- and third-ranking diplomats to international
Burns is a respected veteran who was central to the secret U.S.-Iranian back channel that helped reach an interim nuclear deal with Iran last November. In those meetings Burns helped reassure the Iranians that the White House was committed to trying to complete a deal.
He will sit in on several key meetings Monday, including a planned three-way session with the United States, the Iranians and the European Union.
This week is considered critical in the negotiations. Western officials recognize that major gaps remain in the negotiations, but want to bear down and see if the deal can be completed by next month.
Burns led a group that last week met bilaterally with the Iranians in Geneva to discuss the 5-month-old talks. He is said to have good relations with some of the Iranian team.
The negotiators are seeking a comprehensive deal that would restrain Iran's ability to develop a nuclear weapons capability, in return for a lifting of tough international sanctions on Iran's economy.
Although both sides say they remain committed to the deal-making, the two sides reached an impasse on key issues at the May negotiating session in Vienna. Each has accused the other of failing to accept the compromises that will be necessary for a deal.
Diplomats from several countries have expressed doubts that the group will be able to finish the deal by the current deadline of July 20. If they decide to extend the deadline - by up to six months, as the interim deal provides - they will probably need to negotiate terms for what would amount to a second interim deal, analysts say.
That deal would probably have to include sweeteners for both the Iranians and the West, to provide incentives for hard-liners in Tehran and skeptics in Congress to accept the extension. Some U.S. lawmakers have already signaled they would resist an extension, arguing that Iran is just trying to run out the clock.
But administration allies in the Senate say they are confident such a deal would be accepted in Washington.