Washington, May 12 (IANS): President Barack Obama spoke by telephone with Saudi Arabia's King Salman, one day after Riyadh announced that the monarch will not attend a US summit of Persian Gulf leaders scheduled for later this week.

The White House said that Salman called the president on Monday to "apologise" for not being able to come to Washington this week, Efe news agency reported.

Obama and the monarch reviewed the agenda for the meeting and agreed on the need to work closely to fight collectively in the most effective way against the "threats" facing the Middle East.

The two leaders also discussed the importance of achieving a comprehensive accord between the P5 1 Group of major world powers -- the US, Russia, China, Britain, France plus Germany -- and Iran to limit Teheran's nuclear programme and ensure it remains peaceful.

With regard to the situation in Yemen, where Saudi Arabia has been undertaking airstrikes against Houthi rebels backed by Iran, Obama "congratulated" the king on the ceasefire implemented by the Saudi army there and they agreed on the need to resolve the "urgent humanitarian" situation.

The US administration wants to welcome a large Saudi delegation, said Ben Rhodes, Obama's deputy national security advisor, in a telephonic press conference.

The Saudi government announced on Sunday the decision by King Salman to send Saudi Crown Prince Mohammed bin Nayef bin Abdelaziz, who serves as interior minister, and Defence Minister Mohammed bin Salman to the summit in his place, and the king confirmed that in his telephone conversation with Obama.

Various analysts have interpreted that decision as a slap on Obama resulting from Saudi frustration over the nuclear negotiations with Iran, but the White House has rejected that interpretation.

The two princes are key actors in Saudi security policy, Rhodes said.

They are in charge of the kingdom's security ministries and the administration believed that the necessary people will be around the table at the summit, Rhodes added.

Of the six countries invited to the Wednesday-Thursday summit -- Kuwait, Qatar, Oman, Bahrain, Saudi Arabia and the United Arab Emirates -- only the first two are being represented by their top leaders, the White House confirmed on Monday.

The summit with the countries belonging to the Gulf Cooperation Council (GCC) will begin with a White House dinner on Wednesday and that same day a bilateral meeting between Obama and Salman was planned before the monarch cancelled his visit.

On Thursday, Obama and the GCC leaders will travel to Camp David to hold talks focusing on security, including the fight against terrorism, the conflicts in Iraq, Syria, Yemen and Libya, the nuclear talks with Tehran and Iran's role in the region.