* Critics say Obama preparing to ease sanctions prematurely
* Obama sees unintended consequences in any military action
* Kerry speaks with Netanyahu, notes Israeli concerns
By Matt Spetalnick
WASHINGTON, Nov 14 (Reuters) - President Barack Obama urged skeptical U.S. lawmakers on Thursday to hold off on imposing new sanctions on Iran, saying that if diplomacy fails to curb Tehran's nuclear program, any punitive measures that are eased through negotiations could be "ramped back up."
In his most direct appeal yet for more time to pursue a diplomatic deal with Iran, Obama sought to assuage concerns in Congress and among U.S. allies including Israel and Saudi Arabia that his administration is giving away too much in talks between Tehran and six world powers.
Obama spoke a day after Vice President Joe Biden, Secretary of State John Kerry and other top U.S. officials warned senators that implementing new sanctions could scuttle the delicate negotiations due to resume in Geneva on Nov. 20. Some lawmakers said after the meetings they were not convinced.
"If we're serious about pursuing diplomacy, then there's no need for us to add new sanctions on top of the sanctions that are already very effective, and that brought them (the Iranians) to the table in the first place," Obama told a White House news conference.
"Now, if it turns out they can't deliver, they can't come to the table in a serious way and get this issue resolved, the sanctions can be ramped back up ... and we've got that option," he said.
An initial agreement seemed close last week, when Kerry made an unexpected trip to the talks in Switzerland. But the negotiators failed to reach a deal last weekend and are returning for another round of talks.
Israeli Prime Minister Benjamin Netanyahu warned on Wednesday that a "bad deal" with Iran on its nuclear program could lead to war. His aides challenged U.S. assertions that Iran was being offered only limited relief from sanctions.
OBAMA ANSWERS CRITICS
Obama sought to answer critics who accuse the administration of preparing to ease sanctions prematurely.
He said that in return for Iran's agreement in a "short-term, phase-one" deal to halt its nuclear advances, "we would provide very modest relief at the margins of the sanctions that we've set up."
"But importantly, we would leave in place the core sanctions that are most effective and have most impact on the Iranian economy, specifically oil sanctions and sanctions with respect to banks and financing," he added.
Obama said this would give world powers a chance to test how serious Tehran is about negotiating a final deal to dispel Western suspicions that it wants to develop a nuclear weapon, something Tehran denies it is seeking.
"It also gives us an assurance that if it turns out six months from now that they're not serious," he said, "we can dial those sanctions right back up."
Obama reiterated that he was leaving "all options on the table" for dealing with Iran - diplomatic code for possible military action. But he warned of "unintended consequences" from any military conflict.
"No matter how good our military is, military options are always messy, are always difficult, always have unintended consequences - and in this situation are never complete in terms of making us certain that they (the Iranians) don't then go out and pursue even more vigorously nuclear weapons in the future," he said.
Senior lawmakers expressed frustration on Wednesday with the Obama administration's call to delay new sanctions.
The Republican-controlled House of Representatives passed its version of a new sanctions bill on July 31, just days before Iran's new president, Hassan Rouhani, took office. Rouhani was elected in June on a platform of conciliation, saying he wanted to ease Iran's international isolation.
Senators have been debating behind closed doors their version of the bill, which could slash Iran's oil exports to no more than 500,000 barrels a day and reduce the ability of the Obama administration to waive sanctions.
Kerry said earlier on Thursday he understands Israel's concerns over Iran's nuclear program and that the two allies share the same goal although they differ in tactics.
Kerry told MSNBC's "Morning Joe" program that he had just spoken with Netanyahu by phone before appearing on the television network.
"We're having a very friendly and civil conversation about this," Kerry said. "I respect completely his deep concerns - as a prime minister of Israel should have - about the existential nature of this threat to Israel. We understand that." (Additional reporting by Steve Holland, Jeff Mason, Roberta Rampton, Susan Heavey, Patricia Zengerle; Editing by Will Dunham)