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King Salman of Saudi Arabia will not be attending a landmark summit hosted by President Barack Obama, amid worries over US-Iran nuclear negotiations. 

Obama had invited six Gulf Arab leaders to the presidential retreat at Camp David, seeking to shore up wavering trust while Washington negotiates with Tehran. 

Only two heads of state are now expected to attend the Thursday meeting. 

Saudi Arabia's embassy in Washington said Sunday that recently named Crown Prince Mohammed bin Nayef would instead lead the Saudi delegation to the meeting.  

The king's son, Deputy Crown Prince Mohammed bin Salman who has driven recent military operations in Yemen, will also attend.  

Saudi Foreign Minister Adel al-Jubeir said Salman would miss the meeting "due to the timing of the summit, the scheduled humanitarian ceasefire in Yemen and the opening of the King Salman Centre for Humanitarian Aid," according to the embassy statement.  

Oman's Sultan Qaboos has been ill, and diplomats said Muscat will be represented by the deputy prime minister. 

Abu Dhabi's Crown Prince Sheikh Mohammed bin Zayed al-Nahayan will attend, according to diplomats, as United Arab Emirates President Sheikh Khalifa bin Zayed al-Nahyan is also unwell.   

As late as Friday, US officials said they had expected Salman to come to Washington, before learning of the change in plan.  

"This is not in response to any substantive issue," insisted one senior US administration official.  

Bahrain's King Hamad bin Issa al-Khalifa will also miss the meeting, officials indicated Sunday, with the crown prince coming instead. 

That means Obama will likely meet only the leaders of Kuwait and Qatar.  

Iran worries

The summit comes amid unease over Washington's nuclear talks with Tehran and perceived US disengagement in the region under Obama's administration.  

US officials stress there is no broader detente or a de facto blessing of Iran's destabilising support for proxy groups in the region.   

They also point to the presence of the US 5th Fleet in Bahrain and a military base and command centre in Qatar as evidence of sustained engagement.  

But Gulf states look anxiously at Iran's growing influence in Iraq, Syria, Lebanon and Yemen.  

"In the last two years….we have seen an increase in Iranian activity in the region, we are seeing more arms shipments, more money…." said a Gulf diplomat.  

No permission needed

Already calculations about US engagement are changing actions on the ground.  

"Our Gulf allies aren't waiting for US action anymore, they don't think it's coming soon enough, or strong enough. Or at all," said Lori Plotkin Boghardt, a former CIA analyst now at the Washington Institute for Near East Policy.   

Officials from the region and beyond point to the Arab-led intervention in Yemen as a harbinger of things to come.  

"Yemen shows you that we will not wait for permission anymore," one Gulf official said.  

But Yemen brings significant problems for Obama, which he will not want to see repeated elsewhere.  

His administration has misgivings about Saudi Arabia's military action to "degrade" Houthi militia, which have cut through government forces.  

They have pressed Saudi Arabia to ease an air campaign that appears to have had a limited impact beyond destroying missiles that could reach the Saudi homeland, and which may have simply escalated the conflict. 

Around 1,200 people have been killed in the Yemen war.  

US support for a "humanitarian pause" in Saudi military operations, and backing for talks leading to a unity government and prising away former Yemen president Ali Abdullah Saleh from the houthis could all help Saudi Arabia.  

Gulf leaders will also come with a wish list of weapons to counter the threat from Iran.  

Officials say the Gulf leaders will seek advanced US weapons systems including F-35 stealth fighters to help establish an Arab "qualitative military edge" as Iran grows its advanced missile capabilities.  

Russia recently agreed to sell Iran the S-300 air defence missile system capable of shooting down fighters currently used by Gulf states.  

US backing for Saudi or Emirati nuclear programmes are off the table, but missile defence is not.  

But diplomats warn that with existing US military commitments to Israel, Japan, Egypt and Turkey, determining who gets what weapons and when, while maintaining a favourable military balance, will be fraught.