At least eight civilians were killed and 20 wounded Monday in explosions sparked by Saudi-led air strikes on rebel arms depots in the Yemeni capital, a medical source said.

The strikes on the Mount Noqum base on the eastern outskirts of Sanaa sent munitions and shrapnel flying into adjacent residential neighborhoods, witnesses said.

Several Americans have been detained in Yemen, a State Department official said Sunday, amid reports that at least four U.S. citizens are being held by Shiite rebels.

The Washington Post reported that the Americans were believed to be held by the Huthi militia in a prison near the capital Sanaa, and that U.S. efforts to secure their release had faltered, hampered by the fact that Washington has no direct links to the rebels.

A U.N.-chartered ship loaded with humanitarian supplies bound for Yemen was targeted by shelling as it approached the country's main southern port of Aden on Sunday, an official said.

The provincial government official, who spoke on condition of anonymity, blamed Shiite Huthi rebels who control several neighbourhoods in Yemen's second city for not allowing the ship to dock at Aden's port, which is controlled by pro-government fighters.

Saudi Arabia and Egypt said Sunday they shared a "common vision" on the war in Yemen, where a Riyadh-led coalition is waging a bombing campaign against Iran-backed rebels.

The coalition, which includes Egypt, began air raids in March targeting Huthi Shiite rebels and allied forces loyal to ousted president Ali Abdullah Saleh in Yemen.

Security in Saudi Arabia is "under control", the kingdom's crown prince, who led a previous crackdown on al-Qaida, said Sunday after the Islamic State group claimed a fatal mosque attack.

"Incidents such as this will not destabilize us. We have been through bigger ones," said Crown Prince Mohammed bin Nayef, who is also the interior minister.

Prime Minister Haider al-Abadi sought to reassure Saudi Arabia Sunday that Iraq is not a "gateway for Iran" and will not take part in regional conflicts between the two countries.

"Iraq is not a gateway for Iran," Abadi said in an interview with Iraqiya state TV.

Human Rights Watch on Sunday published new evidence alleging a Saudi-led coalition is using internationally banned cluster bombs in Yemen, urging it to stop such attacks that were harming civilians.

The New York-based watchdog said it documented the use of three types of cluster munitions in Yemen, where Saudi-led warplanes have pounded positions of Shiite rebels and allies loyal to former president Ali Abdullah Saleh since March 26.

Shelling from Yemen has killed a Saudi border guard and wounded seven others, the latest casualties along the kingdom's southern frontier, the interior ministry said on Sunday.

The attack happened at 6:30 pm (1530 GMT) Saturday when shells "from across the border with Yemen" hit their patrol, the ministry said in comments carried by the official Saudi Press Agency.

An Islamic State-affiliated group in Saudi Arabia has urged more attacks against Shiites, after IS claimed two bomb attacks on mosques in the kingdom.

An IS suicide bomber on Friday blew himself up at the entrance of a mosque in Eastern Province, killing three people, exactly seven days after the jihadist group attacked another mosque in the oil-rich region.

Saudi-led coalition warplanes pounded rebel positions across Sanaa overnight, only hours after the UN envoy to Yemen arrived in the capital to discuss stalled Geneva peace talks, residents said Saturday.

The escalation came as neighboring Oman, which enjoys good relations with Tehran and is a member of the Gulf Cooperation Council (GCC) alongside Saudi Arabia, hosted representatives of the Shiite Huthi rebels and delegates from Iran.