UN Secretary-General Antonio Guterres on Friday urged the United States and Iran to avoid further escalation of the situation, said his spokesman.

"The secretary-general's message to all the parties involved is to avoid anything that would escalate the situation further. As he put it: to have nerves of steel," Stephane Dujarric, the spokesman, told reporters.

"The secretary-general firmly believes in the need for dialogue between the parties involved as probably the best way to defuse tension and to avoid any escalation."

Dujarric said the secretary-general and his team are following the situation closely and the world body has been in contact with the parties at various levels.

Asked whether Guterres would meet US President Donald Trump at the Group of 20 summit in Osaka, Japan, next week, Dujarric said the secretary-general will use every possible opportunity to pass the same message to all the parties, which is to avoid any escalation.

He could not confirm that such a meeting will happen, saying the UN chief's bilateral schedules are being worked out.

The already tense relations between Tehran and Washington flared up suddenly after Iran shot down a large US drone on Thursday.

Iran said the drone, a US Navy RQ-4A Global Hawk, violated its airspace, while the United States said it was flying over international waters when it was downed.