Image copyright AP Image caption The two planes are now sitting on the tarmac at Hartsfield-Jackson International Airport

Police have searched two airliners at Atlanta's airport after "credible" bomb threats, US officials say.

Airport spokesman Reese McCranie said the threats were received against Delta and Southwest flights coming from Portland and Milwaukee respectively.

He said both planes had landed safely and had been evacuated.

Two US fighter jets escorted the planes into the Hartsfield-Jackson International Airport. The airport later said no bombs were found.

In a tweet, airport officials wrote: "All clear for both aircraft & normal airport operations have resumed. Thank you for your patience. Safety & security are our top priorities."

The bomb threats were made online - reportedly on Twitter.

Image copyright AP Image caption The passengers from the two flights were evacuated

A bomb squad and sniffer dogs were involved in the police search after the planes landed.

Southwest said in a statement: "Our top priority is the safety of our customers and employees. We cannot comment on the nature of the security situation.''

Reuters quoted the company as saying that 86 passengers were on board the flight, and that they were being re-screened.

There was no immediate comment from Delta officials.

The busy Hartsfield-Jackson International Airport is home to Delta, serving nearly 100 million passengers.

This is not the first airport bomb scare in the US this week.

On Monday, New York's JFK was at the centre of another threat, involving at least one aircraft. No bomb was found.