– President Obama welcomed Saudi Arabian leaders to the White House on Wednesday amid strains with the kingdom over his pursuit of a nuclear deal with Iran.

As he opened their Oval Office meeting, Obama said the U.S. and Saudi Arabia were building their relationship "during a very challenging time." Beyond the Gulf nation's worries about Iran, Saudi Arabia and other countries are deeply concerned about the rise of the Islamic State in Iraq and the Levant and instability in Yemen.

Obama made no mention of the nuclear negotiations, which will be at the forefront of discussions with leaders from the Persian Gulf region at Camp David on Thursday.

The president also hosted a dinner Wednesday for representatives from Saudi Arabia, Qatar, the United Arab Emirates, Kuwait, Oman and Bahrain.

But Obama's separate meeting with the Saudis underscores the desert kingdom's critical role. Saudi Arabia has been among the strongest critics of the president's Iran overtures and worries not only about Tehran's nuclear ambitions, but also its meddling throughout the region.

Obama had planned to meet Wednesday with Saudi King Salman. But the kingdom abruptly announced over the weekend that the king would not travel to Washington.

Both the White House and Saudi officials insisted Salman was not snubbing Obama. Still, his decision to skip a visit to the White House and a summit at the presidential retreat raised questions about the meetings' effectiveness.

The White House is expected to offer the Gulf nations more military assistance, including increased joint exercises and coordination on ballistic missile systems. The package of assistance would be aimed at reassuring the region that the U.S. will guard its security against potential Iranian aggression.

Some Gulf nations wanted Obama to commit to a formal defense treaty, but U.S. officials have told leaders that the president will not agree to such a measure.

The Saudi king isn't the only head of state skipping the meetings in Washington. The heads of the United Arab Emirates and Oman have had health problems.

Bahrain's King Hamad bin Isa Al Khalifa will be in Britain Thursday to attend an event at Windsor Castle and meet with Queen Elizabeth II.

The parties planned to spend Thursday at Camp David, the presidential retreat in Maryland's Catoctin mountains, discussing Iranian nuclear talks and Tehran's reputed support of terrorism.

The U.S. and five other nations are trying to reach an agreement with Iran by the end of June to curb its nuclear ambitions in exchange for relief from international economic sanctions. The Gulf nations fear that an influx of cash will only facilitate what they see as Iran's aggression.

The White House says a nuclear accord could clear the way for more productive discussions with Iran about its reputed terror links. The U.S. has criticized Iran's support for Hezbollah, as well as terror attacks carried out by Iran's Quds Force.