On Saturday, Iranians raised blood-red "flags of revenge" over the minarets at the Jamkaran Mosque in the holy city of Qom. In Shia Islam, the flags symbolize blood spilled unjustly and serve as a call to avenge the person who is slain.

In a tweet Sunday afternoon, Trump said: "These Media Posts will serve as notification to the United States Congress that should Iran strike any US person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner."

Trump tweeted on Saturday: "They attacked us, & we hit back. If they attack again, which I would strongly advise them not to do, we will hit them harder than they have ever been hit before!"

Iraq's parliament called on Sunday for US and other foreign troops to leave the country amid fears of a wider Middle East conflict.

NATO ambassadors will gather on Monday in Brussels for an urgent meeting convened by the head of the military alliance to discuss the situation in the Middle East.

Governments around the world also have warned their citizens to be on heightened alert as a result of the tensions between Iran and the United States.

In a statement issued Sunday, the Chinese embassy in Washington said Chinese citizens in the US should "closely watch the security situation, stay alert and take safety precautions, be cautious before going to public places".

In New York City, Mayor Bill de Blasio tweeted on Friday: "No one has to be reminded that New York City is the number one terror target in the United States. We're taking escalation in the Middle East seriously — and I have absolute faith in the NYPD to protect this city and keep every New Yorker safe."

US Secretary of State Mike Pompeo said Washington would target any Iranian decision-makers it chose if there were further attacks on American interests by Iranian forces or their proxies.

The European Union, Britain and Oman urged Washington and Teheran to make diplomatic efforts to defuse the crisis.

EU foreign policy chief Josep Borrell spoke with Iran's foreign minister by phone to work to de-escalate the situation and invited him to Brussels to discuss ways of preserving world powers' nuclear deal with Iran.

The permanent members of the United Nations Security Council — China, the United States, the United Kingdom, Russia and France — along with Germany and the European Union agreed to the framework deal.