Reuters| Jonathan Landay, Matt Spetalnick and Parisa Hafezi: Instead of tearing up the Iran nuclear deal, the Trump administration is exploring how to tighten its enforcement and renegotiate key terms, but it may prove impossible to get other major powers and Iran to consider revising the agreement.

Under the 2015 accord between Tehran and six world powers, Iran agreed to curb its nuclear program in exchange for relief from some U.S., European and U.N. economic sanctions. Many experts consider it a success so far.

The Trump administration this week signaled a harder but ambiguous line toward Iran by putting the Islamic Republic "on notice" after an Iranian ballistic missile test, and then by imposing economic sanctions on 13 individuals and 12 entities on Friday..

The options the administration is considering include insisting the Vienna-based International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, get tougher policing Iran's compliance, including demanding access to military sites, according to two sources familiar with the matter.

"The basic idea is that the IAEA has to be granted access," said one source with knowledge of the matter, acknowledging it would be difficult for the U.S. administration to win support for military site inspections from the other 34 countries on the IAEA board of governors.

The U.S. also would seek to remove "sunset" provisions from the pact that allow some restrictions on Iran's nuclear program to start expiring in 10 years, something critics consider the deal's biggest flaw, the two sources said.

In addition, the administration could press the agency to report more information on Iran's compliance with the nuclear pact, one of the sources said, noting the agency has cut back on reporting some data, such as how much low-enriched uranium Iran is stockpiling.

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The administration is also weighing a push to tighten the rules of the Procurement Working Group, a panel set up under the accord to make it harder for Iran to win approval to import sensitive technologies and materials that might be used for nuclear purposes, said one of the sources.

There are concerns that the Obama administration was too lenient in agreeing to Iranian purchases of nuclear-related materials, including 116 metric tons of natural uranium, for which Tehran has no apparent need, one source said.

Getting the United States' partners in the nuclear deal, a group known as the P5+1 that includes Britain, China, France, Germany and Russia, into such a negotiation is likely to be extremely difficult, let alone bringing the Iranians along.

"It seems to be a long shot," said a P5+1 source on the odds of getting the group into a new negotiation aimed at tightening up the nuclear deal, formally called the Joint Comprehensive Plan of Action (JCPOA) or at creating a sort of "JCPOA II."

Iranian officials have rejected the idea of renegotiating the deal, and Trump's stance could weaken the hand of those in Tehran who have been willing to negotiate with the West, said a former senior Iranian official.

"Hardliners will use Trump's stance to strengthen their position," said a former senior Iranian official close to President Hassan Rouhani. "They have always been against Rouhani's d�tente policy toward West."

(Writing By Arshad Mohammed; Editing by John Walcott and Sandra Maler)