Iran's leaders are promising "revenge" and "harsh vengeance" over the US killing of Major General Qassem Soleimani in Iraq. 

Soleimani, head of the Islamic Revolution Guard Corps' (IRGC) elite Quds Force, was assassinated in a strike on Baghdad International Airport on Thursday (local time), along with militia leader Abu Mahdi al-Muhandis.

Iran's Supreme Leader Ayatollah Ali Khamenei has since described Soleimani as "the international face of resistance" and his death is being commemorated with three days of public mourning in the nation.

The Pentagon confirmed its responsibility for the attack, saying president Donald Trump directed it himself.

• Pentagon says US airstrike killed powerful Iranian general, at direction of President Trump
• US kills top Iranian military commander in attack at Baghdad airport

"Harsh vengeance awaits the criminals that got [Soleimani's] and other martyrs' blood on their evil hands in [the attack]," Khamenei said, according to the BBC.

Iranian president Hassan Rouhani reportedly added: "Without a doubt, the great Iranian nation and other free nations in the region will take revenge on criminal America for this ghastly crime."

On Twitter, a Farsi phrase meaning "harsh vengeance" is trending in Iran, and at the time of publishing, 'Iran', 'World War 3' and '#WWIII' are the top three trends globally on the social media platform.

Hessameddin Ashena, an adviser to Rouhani, was also quick to warn of retaliation.

"Trump, through his gamble, has dragged the US into the most dangerous situation in the region," Ashena wrote on the social media app Telegram.

"Whoever put his foot beyond the red line should be ready to face its consequences."

Iranian television also described the airport attack as "the biggest miscalculation by the US". 

"The people of the region will no longer allow Americans to stay," the country's state broadcaster said.

Trump's only response since to the attack has been to tweet an image of the US flag.