A Shiite parliamentarian in Kuwait on Monday demanded to grill the foreign minister over the Gulf state's participation in Saudi-led air strikes against Yemen's Shiite Huthi rebels and their allies.

"Kuwait's air force has taken part in the military operations without parliament being informed," Abdulhameed Dashti said.

Yemeni rebels said on Monday they shot down a fighter jet as Saudi-led coalition air strikes intensified a day ahead of a hoped-for five-day humanitarian truce.

Morocco announced that one of its warplanes taking part in operations against the rebels was missing along with its pilot, and that a search was underway.

Saudi-led warplanes pressed air strikes against rebel positions in Yemen early Monday, 36 hours before a scheduled five-day pause to allow the delivery of much-needed humanitarian aid, witnesses said.

The coalition pounded the Shiite Huthi rebel stronghold of Saada in the northern mountains for a third straight night after declaring the whole province a military target on Friday.

Newly crowned Saudi King Salman has refused an invitation to attend a landmark summit hosted by President Barack Obama, amid angst over U.S.-Iran nuclear negotiations. 

Obama had invited six Gulf kings, emirs and sultans to the presidential retreat at Camp David, seeking to shore up wavering trust while Washington negotiates with regional power Tehran.

The Saudi-led air war on Yemen was launched to foil a plot by a "sectarian group" to undermine security in the region, King Salman said on Sunday.

The Saudi monarch was defending the military operation Riyadh has led against Yemen rebels since March 26, in an address read to clerics on his behalf by an adviser in the Muslim holy city of Mecca.

Yemen ceasefire efforts gathered pace Sunday after more than six weeks of Saudi-led air strikes, with rebels saying they would respond "positively" and their allies accepting a U.S.-backed truce plan.

The renegade troops, who helped the Shiite Huthi rebels seize much of the country, said they had agreed to the five-day humanitarian truce that Riyadh has offered from Tuesday.

Unidentified gunmen shot dead a policeman in the capital of Saudi Arabia on Friday, a police spokesman said.

Gunmen in a car opened fire at a patrol in southern Riyadh, killing one of its members, a spokesman for Riyadh region police said in a statement carried by the SPA state news agency.

Turkey on Friday denied it had struck a secret new pact with Saudi Arabia to back Islamist rebels in Syria in a drive to bring down Syrian President Bashar Assad.

Reports have suggested that Riyadh and Ankara agreed a special pact to militarily and financially support Islamist rebels in Syria in an all-out effort to oust Assad from power. 

Saudi Arabia Friday announced a humanitarian ceasefire in Yemen starting May 12, even as it stepped up retaliatory air strikes on Shiite Huthi rebels in the north of the country.

"We have made a decision that the ceasefire will begin this Tuesday, May 12, at 11:00 pm and will last for five days subject to renewal if it works out," Saudi Foreign Minister Adel al-Jubeir said at a meeting of Gulf ministers in Paris.

U.S. Secretary of State John Kerry met Gulf foreign ministers in Paris on Friday ahead of a summit in which President Barack Obama will seek to reassure the Arab monarchies over his Iran policy.

Fresh from a trip to Riyadh where he urged a ceasefire in Yemen, Kerry met foreign ministers from the six members of the Gulf Cooperation Council: Qatar, Kuwait, Bahrain, Oman, the United Arab Emirates and Saudi Arabia.