By Matt Spetalnick

WASHINGTON (Reuters) - President Barack Obama will assure Saudi King Salman of the U.S. commitment to help counter any Iranian security threat, White House officials said on Wednesday, despite concern among Gulf allies that a new nuclear deal could empower Tehran in the region.

Obama, hosting Salman on Friday on the king's first U.S. visit since ascending to the throne in January, will seek to allay the fears of Washington’s most important Arab partner that the lifting of sanctions on Iran would allow it to act in destabilizing ways.

The White House talks will come less than two weeks before a possible U.S. congressional vote on the nuclear deal between six world powers and Iran, Riyadh’s regional rival. The Obama administration wants to use the visit to shore up relations with Saudi Arabia after a period of tensions.

“We understand that Saudi Arabia has concerns about what Iran could do as their economy improves from sanctions relief,” Ben Rhodes, Obama’s deputy national security adviser, told reporters in previewing the visit.

He said the United States believed Iran would use much of its assets, which will be unfrozen under the deal reached in July that also puts curbs on Tehran's nuclear program, to improve its battered economy.

Rhodes acknowledged there was a risk that Tehran could spend those funds on “nefarious activities”. But he said Obama would make clear the United States would do “everything that we can” to counter any Iranian threats to its neighbors.

Gulf Sunni Arab leaders have accused Shi’ite Iran of fomenting sectarian violence in places like Yemen, Syriaand Iraq. At the same time, some critics have blamed the Gulf states for contributing to the regional conflicts.

Rhodes said Salman is expected during his visit to “express a comfort with the Iran deal as it relates to the nuclear issue” but reiterate other Saudi concerns about Tehran.

The Obama administration is focused on providing assistance that the president promised when he hosted a Gulf Arab summit at Camp David in May, including helping them to integrate ballistic missile defense systems and beef up cyber and maritime security.

Salman skipped the Camp David summit, a move widely seen as a diplomatic snub over Obama’s Iran strategy – though both governments denied that interpretation.

Obama and Salman, leader of the world's largest oil exporter, would also discuss global energy markets and address the conflicts in Yemen and Syria, the White House said.

The officials said Obama would express concern to Salman about civilian casualties in Yemen and urge restraint on all sides. A Saudi-led coalition launched a bombing campaign in March against Iran-allied Houthi rebels. TheUnited States is providing limited logistical support for the air assault.

(Additional reporting by Julia Edwards; Editing by Ken Wills)