Tens of thousands of Saudi Shiites gathered Monday for a mass funeral for the victims of a mosque bombing authorities called an attempt by Sunni extremists to sow sectarian strife.

Mourners from the kingdom's minority crowded the streets in the mainly Shiite Qatif district of Eastern Province to show their respect for the 21 dead, who included two children.

They are angry and grief-stricken, but Saudi Arabia's minority Shiites refused on Sunday to be provoked by a deadly mosque bombing that authorities called an attempt to promote sectarian strife.

The interior ministry confirmed the identity of the suicide bomber who blew himself up inside a Shiite mosque in Eastern Province on Friday, killing 21, and said he had links with the Islamic State jihadist group.

Saudi Arabia's King Salman pledged on Sunday that anyone linked to the killing of 21 minority Shiites will face justice for the "heinous crime."

Salman made the vow in a telegram to Crown Prince Mohammed bin Nayef, who is also the interior minister.

A number of Syrian opposition factions will gather in Cairo next month to form a new coalition as an alternative to an exiled West-backed alliance, officials said Saturday.

More than 200 figures from the armed and civilian opposition factions are to attend the June 8-9 gathering and discuss a roadmap aimed at ending the four-year war in Syria.

United Nations officials in the Horn of Africa port of Djibouti said Saturday they had taken charge of the aid cargo carried by an Iranian boat headed for war-torn Yemen.

The vessel, the MV Shahed, is carrying 2,500 tonnes of aid including flour, rice, canned food, medical supplies and bottled water, all urgently needed in the conflict-wracked state just across the Gulf of Aden from Djibouti.

Regional Shiite power Iran on Friday denounced a suicide bombing at a Shiite mosque in Saudi Arabia that killed and wounded several people.

Official IRNA news agency quoted foreign ministry spokeswoman Marzieh Afkham as "vigorously condemning" the attack, which has been claimed by the Islamic State jihadist group.

Al-Mustaqbal movement leader ex-PM Saad Hariri on Friday condemned as “terrorist” the suicide bombing that rocked a Shiite mosque in the eastern Saudi region of Qatif, noting that it is aimed at “stirring sedition” and that it serves the interests of Yemen's Huthi rebels and other Iran-backed “militias.”

“The objectives of this criminal attack are obvious, because it is part of a vicious cycle seeking to stir sedition among the sons of the kingdom and to jeopardize its stability,” Hariri said in a statement.

Hizbullah on Friday held Saudi authorities “responsible” for a deadly suicide bombing that rocked a Shiite mosque in the kingdom's east, accusing Riyadh of “negligence” and “incitement.”

“We hold Saudi authorities fully responsible for this heinous crime, due to its sponsorship, embracement and support for the criminal murderers at the ideological, journalistic, political, material and operational levels … and due to its negligence in providing protection to its citizens who live in the Eastern Province and its incitement against them,” Hizbullah's media department said in a statement.

An Islamic State group suicide bomber attacked a Shiite mosque in Sunni-ruled Saudi Arabia on Friday, killing and wounding several people in an assault that threatens to fan sectarian tensions.

The bomber struck during the main weekly prayers in Eastern Province, where assailants linked to the Sunni extremist IS killed seven members of the minority Shiite community in November.

Saudi-led coalition warplanes carried out fresh raids on rebel positions in southern Yemen Thursday as pro-government tribesmen advanced on Shiite Huthi strongholds in the north, tribal and army sources said.

On the political front, Iran said it backed U.N.-brokered efforts to relaunch political talks but was opposed to foreign interference in Yemen's conflict, in which it backs the rebels.