Saudi Arabia has sent 30,000 soldiers to its border with Iraq after Iraqi soldiers withdrew from the area, Saudi-owned Al Arabiya television says.
The country aims to guard its 800km border with Iraq, where Islamic State fighters and other Sunni Muslim rebel 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 Dubai-based Al Arabiya said on its website that Saudi troops fanned into the border region after Iraqi government forces abandoned positions, leaving the Saudi frontier unprotected, the Reuters news agency reported.
The satellite channel said it had obtained a video showing about 2,500 Iraqi soldiers in the desert area 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 and the Iraqi government denied the reports.
Lieutenant-General Qassim Atta, an Iraqi army spokesman, said: "This is false news aimed at affecting the morale of our people and the morale of our heroic fighters."
Iraq is in the midst of a conflict with Sunni fighters in the north and west of the country, and has launched an offensive in Tikrit to recapture territory it lost during a rebel advance in June.
Thousands of soldiers, backed by tanks, artillery and aerial cover, have made limited progress in retaking the city, the AFP news agency reported.
The Iraqi government has asked allies for help in tackling the rebellion, but has received a limited response from the US.
Washington has sent 300 military advisers to Baghdad, falling short of Iraqi Prime Minister Nouri al-Maliki's request for weapons, including to speed up delivery of F-16 jets due for delivery later this year.
The Iraqis have instead turned to Russia and reportedly, Iran.
Russia sold Iraq a dozen Sukhoi-25 jets.
The London-based International Institute for Strategic Studies has said three Sukhoi jets shown landing in Iraq in a video released by the defence ministry were probably from Iran.
Iran has pledged to aid Iraq against the rebels, who are motivated, in part, by Iran's alleged influence on the Iraqi government.