US President Donald Trump on Wednesday warned Iran and groups linked to Tehran against attacking US troops or assets in Iraq, citing a possible "sneak attack" but giving no other details.

"Upon information and belief, Iran or its proxies are planning a sneak attack on US troops and/or assets in Iraq. If this happens, Iran will pay a very heavy price, indeed!" Trump said in a post on Twitter.

It was not immediately clear what information Trump was referring to in his tweet, which was posted after he was scheduled to have a 12:00pm (16:00 GMT) intelligence briefing.

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A top Iranian military aide had earlier cautioned the US of consequences of "provocative actions" in Iraq, Iranian news agencies reported.

"We advise US politicians and military to take responsibility for the consequences of their provocative actions (in Iraq)," General Yahya Rahim Safavi said, quoted by the semi-official news agency Tasnim. "Any US action will mark an even larger strategic failure in the current president's record."

Rahim Safavi made the comments hours before Trump's tweet.

At odds for decades, the US and Iran have seen relations deteriorate further in the nearly two years since Trump abandoned Iran's 2015 multilateral nuclear deal and reimposed US economic sanctions on Tehran.

Iran's president said on Wednesday that, with the advent of the coronavirus, the US had missed a historic opportunity to lift sanctions on his country, though the penalties had not hampered its fight against the infection.

On Tuesday, US Secretary of State Mike Pompeo raised the possibility that Washington might consider easing sanctions on Iran and other nations to help fight the epidemic, but gave no concrete sign it plans to do so.

"The United States lost the best opportunity to lift sanctions," Hassan Rouhani said in a televised cabinet meeting. "It was a great opportunity for Americans to apologise ... and to lift the unjust and unfair sanctions on Iran."

Upon information and belief, Iran or its proxies are planning a sneak attack on U.S. troops and/or assets in Iraq. If this happens, Iran will pay a very heavy price, indeed! — Donald J. Trump (@realDonaldTrump) April 1, 2020

The coronavirus has killed more than 3,000 people in Iran with confirmed infections close to 48,000, making it the worst-hit country in the Middle East and prompting China and the United Nations to urge the US to ease sanctions.

"Americans could have used this opportunity and told the Iranian nation that they are not against them," Rouhani said. "Their hostility (towards Iranians) is obvious."

Trump has adopted a "maximum pressure" policy on Iran aimed at persuading Tehran to negotiate a broader deal that further constrains its nuclear programme, limits its missile programme and curbs its use of proxy forces in Iraq, Yemen and Lebanon.

Washington has offered humanitarian assistance to its longtime foe. But Iran's top authority Supreme Leader Ayatollah Ali Khamenei has rejected the offer.

People wearing protective clothing carrying the body of a victim who died after being infected with the new coronavirus at a cemetery just outside Tehran, Iran [Ebrahim Noroozi/AP Photo]

Although Iranian authorities have said US sanctions had hindered its efforts to curb the outbreak, Rouhani said, "the sanctions have failed to hamper our efforts to fight" against the coronavirus outbreak.

"We are almost self-sufficient in producing all necessary equipment to fight the coronavirus. We have been much more successful than many other countries in the fight against this disease."

Several countries, including the United Arab Emirates, China, the UK, France, Qatar and Turkey, have sent shipments of medical supplies, including gloves and surgical masks, to Iran.

In the first transaction conducted under a trade mechanism set up to barter humanitarian goods and food after Washington's withdrawal from the nuclear deal, Germany said on Tuesday that France, Germany and the UK had exported medical goods to Iran.