DUBAI (Reuters) - Saudi Arabia deployed 30,000 soldiers to its border with Iraq after Iraqi soldiers abandoned the area, Saudi-owned al-Arabiya television said on Thursday, but Baghdad denied this and said the frontier remained under its full control.

The world's top oil exporter, Saudi Arabia shares an 800-km (500-mile) border with Iraq, where Islamic State insurgents and other Sunni Muslim militant groups seized towns and cities in a lightning advance last month.

King Abdullah has ordered all necessary measures to protect the kingdom against potential "terrorist threats", state news agency SPA reported on Thursday.

The U.S.-allied kingdom overcame its own al Qaeda insurgency almost a decade ago and is wary of any encroaching new threat from radical Sunni Islamists.

The Dubai-based al-Arabiya said on its website that Saudi troops fanned into the border region after Iraqi government forces withdrew from positions, leaving the Saudi and Syrian frontiers unprotected.

The Iraqi prime minister's military spokesman denied the forces had withdrawn. "This is false news aimed at affecting the morale of our people and the morale of our heroic fighters," Lieutenant General Qassim Atta told reporters in Baghdad. He said the frontier, which runs through largely empty desert, was "fully in the grip" of Iraqi border troops.

The satellite channel said it had obtained a video showing some 2,500 Iraqi soldiers in the desert region east of the Iraqi city of Karbala after pulling back from the border.

An officer in the video aired by al-Arabiya said that the soldiers had been ordered to quit their posts without justification. The authenticity of the recording could not immediately be verified.

(Reporting by Sami Aboudi; Editing by Mark Heinrich)