President Barack Obama plans to travel to Saudi Arabia in March on a mission to smooth tensions with Washington's main Arab ally over US policy on Iran's nuclear program and the civil war in Syria, a newspaper reported.

Obama is preparing to meet with King Abdullah for a summit, the Wall Street Journal reported on Friday, citing unnamed Arab officials briefed on the meetings.

"This is about a deteriorating relationship" and declining trust, said a senior Arab official in discussing the need for the summit, which was pulled together in recent days, the newspaper reported.

A White House spokeswoman declined to comment.

The US and Saudi Arabia have been allies since the kingdom was formed in 1932, giving Riyadh a powerful military protector and Washington secure oil supplies.

Washington's relationship with the Saudis was crucial as the region faced changes and challenges from the transition in Egypt to civil war in Syria. But relations have been tested on a number of fronts.

Members of Saudi Arabia's ruling family threatened a rift with the United States to protest perceived American inaction over Syria's civil war, which has killed more than 100,000 people, as well as the recent US outreach with Iran.

The Sunni Muslim kingdom's regional rivalry with Shi'ite Iran, an ally of Syria, has amplified sectarian tensions across the Middle East.

King Abdullah is also to use the meeting to question Obama on why he decided against airstrikes in Syria, which Saudi and other Arab officials believe strengthened Assad, the newspaper reported.

"The meeting in many ways will get back to basics," said a Saudi official briefed on the meetings, according the Wall Street Journal. "Why did Obama do it the way he did it?"

US and other security officials said earlier this week that "moderate" Syrian rebel factions were receiving light arms supplied by the United States.

The Arab Spring as well as a November agreement between Iran and other world powers that curbs parts of Tehran's nuclear program, has angered Saudi Arabia, along with other Arab states and Israel.

The relationship was also badly shaken by the Sept. 11, 2001, attacks on the US, since most of the hijackers were Saudi nationals, and by the subsequent American invasion of Iraq.

Obama's visit comes after a mission to Riyadh by US Secretary of State John Kerry.-Reuters