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Iran raised a red flag over a mosque hinting impending revenge on the US after Donald Trump ordered the execution of a top Iranian general.

Thousands of mourners chanting "death to America" took to the streets of Baghdad after Trump authorised the killing of General Qassem Soleimani yesterday.

For the first time in history, Iran raised a red flag over the Holy Dome of Jamkarān Mosque believed to be a warning of a battle to come.

The disturbing flag reportedly reads: "Those who want to avenge the blood of Hussein."

Trump said he ordered the killing to prevent a conflict, but Tehran has vowed harsh retaliation - raising fears of an all-out war.

(Image: Twitter) (Image: ISNA/AFP via Getty Images)

Leader of the Islamic Revolution in Iran Imam Sayyed on Friday vowed “severe revenge” in response to the killing of Soleimani, Daily Express reports.

The Foreign Office strengthened its warnings over travel to Middle East nations amid the ratcheting of tensions in the wake of the US's drone strike.

British nationals are advised not to travel to Iraq, apart from essential travel to its Kurdistan Region, while all but essential travel to Iran is warned against.

(Image: AFP via Getty Images) (Image: ABEDIN TAHERKENAREH/EPA-EFE/REX)

The guidance was bolstered on Saturday after the United States announced it was sending nearly 3,000 extra troops to the region.

And the Foreign Office warned that anyone in Iraq outside the Kurdistan Region should consider leaving by commercial means because the "uncertain" security situation "could deteriorate quickly".

Alerts regarding other Middle East nations were also being increased, with calls for citizens to "remain vigilant" in nations including Afghanistan, Israel, Lebanon, Saudi Arabia and the Untied Arab Emirates.

(Image: KHAMENEI.IR/AFP via Getty Images)

(Image: AFP via Getty Images)

Foreign Secretary Dominic Raab said the updated advice was issued due to "heightened tensions in the region" and would be kept under review.

"The first job of any Government is to keep British people safe," he added.

There has been criticism of the US for not giving advanced notice of the attack to the UK, which has hundreds of troops deployed in Iraq.

Former foreign secretary Jeremy Hunt said the failure to notify was "regrettable" because allies should ensure "there are no surprises in the relationship".