The Dubai-based television channel al-Arabiya reported on Thursday that Saudi troops had moved into the border region after Iraqi forces loyal to the government abandoned their positions, leaving frontiers of Saudi Arabia and Syria exposed.

Saudi Arabia's King Abdullah was reported to have ordered troops to take all necessary measures to protect the kingdom from jihadists calling themselves the Islamic State (IS).

Al-Arabiya, a Saudi-owned broadcaster, said it had obtained a video that apparently showed some 2,500 Iraqi troops in a desert region of Iraq, after they were ordered to withdraw from the border. An officer featured in the footage was reported as saying the soldiers had been ordered to quit their positions.

Earlier this week, IS declared a caliphate across a large swathe of northern Iraq and Syria. In a lightning offensive last month, the group swiftly seized predominantly Sunni areas across northern and western Iraq, including the country's second-largest city Mosul. However, the militants' advance towards Baghdad has been slowed, and even reversed, as they reach more resistant Shiite majority areas.

US President Barack Obama discussed the threat posed by the Sunni insurgent group in a telephone call with Abdullah, who last week vowed to act against "terrorist threats" in Iraq.

Obama and Abdullah agreed to cooperate closely on events unfolding across the 500-mile (800-kilometer) border between the two countries.

Iraqi government troops say they are making slow advances in their mission to retake Salaheddin province. The province's governor, Ahmed Abdullah Juburi, said forces were advancing "slowly, because all of the houses and burned vehicles en-route to the state capital city of Tikrit had been rigged with explosives.

Iraqi Prime Minister Nouri al-Maliki - who is accused of monopolizing power and fanning the flames of the insurgency by marginalizing the country's Sunni minority - on Thursday extended an amnesty aimed at undermining support for IS among tribal groups. Maliki has faced growing criticism amid political disunity that has prevented a new government being formed in Iraq.

Meanwhile, in Syria, a succession of towns along the River Euphrates fell to IS fighters, as rebels groups affiliated to tribes in the area gave up their battle with the jihadists.

rc/dr (Reuters, dpa)