JERUSALEM, Jan 15 (Reuters) - The United States and Israel have postponed an upcoming joint military exercise until later in the year, but sources in both countries denied on Sunday that the move was taken to avoid further escalating tensions with Iran.

The air-defense drill, named Austere Challenge 12, is expected to be the largest exercise between the two allies, who regularly hold joint military maneuvers.

The Pentagon said it was scheduled for the spring, but now would take place in the second half of 2012.

Israeli media reports originally said it was cancelled due to budgetary constraints. But some pundits speculated that the real reason was fear of creating further friction with Iran, which is showing signs of deepening isolation over its refusal to halt nuclear activity.

A U.S. official denied that tensions with Iran were a factor and an Israeli security source cited logistical problems. Both sources spoke on condition of anonymity.

It's for a host of reasons, mainly logistical, but not the reason you cited (tensions with Iran), the Israeli source said.

Pentagon spokesman Navy Captain John Kirby played down the delay, saying it was not uncommon for routine exercises to be postponed.

There were a variety of factors at play in this case, but in general, leaders from both sides believe that optimum participation by all units is best achieved later in the year, Kirby said.

Israel sees the makings of a mortal threat in Iran's uranium enrichment and missile projects, and the Jewish state's leaders have not ruled out the use of military force to stop it.

Iran says its nuclear program is solely for peaceful purposes.

Tensions between Washington and Tehran have risen in recent weeks after U.S. President Barack Obama signed a bill on New Year's Eve that, if fully implemented, would make it impossible for most countries to pay for Iranian oil.

Iran has threatened to close the Strait of Hormuz, the world's most important oil shipping lane, if sanctions prevent it from exporting oil. The United States has said it will not tolerate such a move.

General Martin Dempsey, the top U.S. military officer, is due to travel to Tel Aviv for talks later this week in which Iran is certain to be one of the topics of discussion. It will be Dempsey's first trip there since becoming chairman of the Joint Chiefs of Staff in September.

In a Nov. 30 interview with Reuters, Dempsey said he did not know whether Israel would alert the United States ahead of time if it decided to take unilateral military action against Iran. He also acknowledged differences in perspective between the United States and Israel over the best way to handle Iran and its nuclear program. (Reporting by Dan Williams; Additional reporting by Phil Stewart in Washington; Writing by Ori Lewis; Editing by Peter Graff and Stacey Joyce)