(Adds comment by U.S. secretary of state)
TIKRIT, Iraq, June 22 (Reuters) - Iraqi and U.S. military spokesmen denied on Saturday that U.S. forces were preparing to evacuate hundreds of staff of Lockheed Martin Corp and Sallyport Global from an Iraqi military base where they work as contractors.
Four separate military sources, who spoke to Reuters on condition of anonymity, said on Friday the contractors from the two firms had been preparing to leave Balad military base, which hosts U.S. forces some 80 km (50 miles) north of Baghdad, over "potential security threats", without saying what those threats might be.
One of the military sources and a fifth security source later said the evacuation had been called off early on Saturday after U.S. and Iraqi military personnel assured the companies that safety measures would be stepped up around the base.
Three mortar shells landed on Balad last week, the first of several unclaimed attacks in recent days targeting bases in Iraq hosting U.S. forces and a site used by a U.S. energy firm.
Iraqi and U.S. military spokesmen denied there were any plans for an evacuation in the first place, which the sources disclosed at a time of rising regional tensions between the United States and Iraq's eastern neighbour Iran.
"There are no plans at this time to evacuate any personnel from Balad ... Should there be increased threats to our people, the U.S. Air Force will put measures in place to provide the protections required", Air Force Colonel Kevin Walker said in a statement.
Secretary of State Mike Pompeo said in a statement: "The United States is not evacuating personnel from Balad Air Base."
Iraq's military spokesman said Iraq "protects the safety of our fighters and American advisers and trainers".
A security source familiar with one of the company's operations said its management informed staff supervisors early on Saturday that the evacuation plan for both companies, which was being prepared until Friday, had been halted.
The companies had received assurances from U.S. military personnel at the base that they would step up security at Balad as well as around the base, including aerial surveillance, to ensure the safety of their contractors, the source said.
One of the military sources at Balad said Iraqi commanders had also assured U.S. military officials they would ensure the safety of foreign staff at the base in case of future attacks.
U.S. forces would be allowed to help secure the outside perimeter of the base, a task normally only assigned to Iraqi forces, as part of efforts to step up safety measures for contractors, the source said.
The U.S. coalition in Iraq declined to comment on the additional security measures.
A spokeswoman for Lockheed in the Middle East said: "We are not evacuating Lockheed Martin employees right now from Balad Air Base." She did not say whether any other evacuation was being prepared.
Lockheed and Sallyport did not immediately respond to requests for further comment later on Saturday.
The mortar attack on Balad last week caused no casualties, the military said. No one has claimed responsibility for the attack.
That incident was followed by attacks on two other military installations hosting U.S. forces, near Baghdad and in the northern city of Mosul.
U.S. energy giant ExxonMobil evacuated at least 21 foreign staff from a site near the southern city of Basra on Wednesday after a rocket also hit that facility, an Iraqi security source said.
One of the military sources, an official with knowledge of Balad base's daily operations said on Friday that the U.S. military informed Iraqi officials they would begin evacuating about half of the 800 employees who work for both companies.
The sources said the evacuation could start at any moment.
"Americans informed us that they will only keep limited, necessary staff who work closely on the maintenance of Iraqi F-16 war planes," one of the Iraqi sources said.
Lockheed Martin began delivering the first F-16s to Iraq in 2014.
Local officials blamed Iran-backed Shi'ite militias for the Basra incident. Iran has not commented on the incidents in Iraq, but has strongly rejected accusations by Washington that it was behind several attacks on oil tankers in the Gulf in recent weeks.
The U.S. Embassy in Baghdad evacuated hundreds of staff last month after Washington cited unspecified threats from Iran to U.S. interests in the region. ExxonMobil evacuated its own staff last month, and had begun returning them to Iraq before the Basra attack.
Washington has ramped up sanctions pressure on Tehran since last year and several violent incidents in the Gulf have been blamed on the rising tensions.
President Donald Trump on Friday said he had called off a U.S. strike on Iran at the last minute.
In Iraq, Tehran backs several Shi'ite Muslim militias which have positions close to U.S. military installations. Representatives of those militias have not publicly commented on the recent incidents.
Sunni militant group Islamic State is also trying to stage a comeback in Iraq and has mostly used hit-and-run insurgency tactics against Iraqi forces in recent months.
Reporting by Ghazwan Hassan and Ahmed Rasheed in Baghdad; writing by John Davison; editing by Alison Williams and G Crosse