Three mortar bombs landed inside Saudi Arabia on Monday close to its northern border with Iraq, where Islamist militants have grabbed land in a lightning advance, officials said.

The mortars caused no casualties but will stoke security fears in Saudi Arabia, which is also facing militants on its southern border with Yemen, where at least 10 people died in an al Qaeda raid into the kingdom on Friday and Saturday.

Authorities in Saudi Arabia, the world's biggest oil producer, said they were still looking into who fired Monday's rounds, which landed near a block of flats outside the northern town of Arar.

Saudi King Abdullah last week said he was stepping up security following the advance in Iraq by the militant Islamic State, which has declared a caliphate on land up to his border.

Saudi authorities fear the gains in Iraq made by Islamic State - an al Qaeda offshoot which has shortened its name from the Islamic State in Iraq and the Levant (ISIL) - could radicalise their citizens.

Six Saudi members of al Qaeda, already wanted by authorities, launched their attack on the town of al-Sharurah, close to the southern border with Yemen on Friday, Interior Ministry spokesman Major General Mansour Turki said in comments carried by state news agency SPA on Tuesday.

Two of the militants grabbed 10 hostages and fought their way into a government building where they blew themselves up on Saturday, he added.

In all, five of the attackers were killed and one captured during clashes with security forces, the spokesman said. Four border guards were also killed along with one of the hostages, he added.

On Monday Al Qaeda in the Arabian Peninsula (AQAP), the Yemeni branch of the militant organisation, released photographs it said were of Friday's attack, showing the launch of Grad rockets and the detonation of a truck bomb at the Wadia border post near Sharurah.

In February, as concerns grew over Saudi citizens joining militant groups in Syria's civil war, King Abdullah decreed that travelling overseas to fight would incur long prison sentences and named ISIL and AQAP as terrorist organisations.

In November, a Shi'ite militia in Iraq said it fired six mortar bombs in the kingdom, into a barren patch of desert on another area along 800-km border.

In May Saudi authorities said they had uncovered a militant group with links to both AQAP and ISIL that was planning attacks inside the kingdom.

On Monday a Saudi court jailed four Saudis for travelling to Iraq to fight, and for connections to al Qaeda, SPA reported.-Reuters