US President Donald Trump on Friday said he called off a planned military strike against Iran because it would have been a disproportionate response to Tehran's downing of an unmanned US surveillance drone, adding that more sanctions have been imposed.

"We were cocked and loaded to retaliate last night on three different sights when I asked, how many will die," Trump wrote in a series of morning tweets. "150 people was the answer from a general ... Ten minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone. I am in no hurry."

Trump said US sanctions imposed on Iran were having an effect and, without mentioning details, said more were added Thursday night.

The downing of the drone - which Washington insists was above international waters but Iran says was within its airspace - has seen tensions between the two countries spike further after a series of attacks on tankers the US has blamed on Tehran.

China has called related parties to exercise restraint to avoid further provocations, to solve the dispute through negotiations.

Iranian officials said Friday that Tehran had received a message from Trump warning that a US attack on Iran was imminent but that he was against war and wanted to talk.

Trump's tweets did not address whether he had sent a message to Iran's leaders but repeated his attacks on the deal secured by his Democratic predecessor Barack Obama. 

Oil prices edged down slightly on Friday following the previous day's surge that saw prices soaring more than six percent.

Iran said Friday it had called in the Swiss ambassador, whose country has represented US interests since the severance of diplomatic relations in the aftermath of the Islamic revolution of 1979, to issue a formal protest. 

Deputy foreign minister Abbas Araghchi provided the ambassador with "indisputable" evidence the drone had violated Iranian airspace, the foreign ministry said.

Araghchi "reiterated that Iran does not seek a war and conflict in the Persian Gulf," but warned, "The Islamic Republic of Iran would not hesitate for a moment to decisively defend its territory against any aggression."

Iranian television later broadcast images of what it said was "debris" of the downed drone recovered from Iran's territorial waters.