Jens Stoltenberg called on Iran to stop its "destabilising activities" (Picture: PA).
The head of NATO says he is "concerned" about Iran's conduct, amid rising tensions.
During a visit to the UK, NATO Secretary General Jens Stoltenberg expressed his concerns about the current situation.
"Iran must stop its destabilising activities, stop supporting terrorist groups and stop, for instance, their missile programme," he said.
The Iranian ambassador in the UK has warned the west not to underestimate his country, as tension with the United States continues.
Hamid Baeidinejad said the Iranian military was prepared for "any eventuality" and that other nations should "not try to test" his country, in an interview with Sky News.
Iran has accused the US of "theatrical manoeuvres".
The US has already sent an aircraft carrier and four B-52 bombers to the region, and is reported to have tens of thousands of American troops on standby to be deployed to the Middle East.
As tensions in the region surge, Foreign Secretary Jeremy Hunt said British officials are worried about the risk of a conflict between the US and Iran which neither side intends.
Mr Hunt told reporters in Brussels: "We are very worried about the risk of a conflict happening by accident, with an escalation that is unintended really on either side but ends with some kind of conflict.
This week, former Royal Navy chief Admiral Lord West said military strikes against Iran would be "an absolute catastrophe".
US president Donald Trump denied a report that his administration has updated plans to send more than 100,000 troops to counter Iran if necessary.
But he added: "Would I do that? Absolutely."