WASHINGTON (AP) -- President Barack Obama welcomed Saudi Arabian leaders to the White House 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 and instability in Yemen.

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

The president also was hosting 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 one on one Wednesday with Saudi King Salman. But the kingdom abruptly announced over the weekend that the king would no longer 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 rare summit at Obama's presidential retreat raised questions about the effectiveness of the meetings.

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 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.

By The Associated Press