WASHINGTON (NYTIMES) - More than 2,300 years ago, the Persian capital of Persepolis was burned by a foreign warrior in a fatal blow to the empire and its rich heritage.

The ruins of the ancient city, in modern-day south-west Iran, could now be on United States President Donald Trump's target list of 52 sites he has threatened to attack as tensions escalated between Washington and Teheran.

Mr Trump did not identify which places the US might strike, as he warned on Twitter that he would order - 52 in all, one for each American who was held hostage for the duration of the Iranian Revolution takeover of the US Embassy in Teheran in 1979.

But he said last Saturday (Jan 4) that some of the sites were "very high level & important to Iran & the Iranian culture".

"Those targets, and Iran itself, will be hit very fast and very hard," he added. "The USA wants no more threats!"

On Sunday, he maintained the right to "quickly & fully strike back, & perhaps in a disproportionate manner" should Iran strike any American person or target.

Later that day aboard Air Force One, he told reporters flying with him back to Washington that "they're allowed to kill our people".

"They're allowed to torture and maim our people," he added. "They're allowed to use roadside bombs and blow up our people. And we're not allowed to touch their cultural site? It doesn't work that way."

Even before those comments, the Iranians had reacted with fury.

Earlier on Sunday, Iranian Foreign Minister, Mohammad Javad Zarif shot back: "A reminder to those hallucinating about emulating ISIS war crimes by targeting our cultural heritage: Through millennia of history, barbarians have come and ravaged our cities, razed our monuments and burned our libraries. Where are they now? We're still here, & standing tall."

It was the latest salvo in the war of words that has threatened to spill over into military action since a US military strike last Friday that killed Major-General Qassem Soleimani, Iran's top security and intelligence commander, while he was visiting Iraq. The Trump administration has said the strike was necessary to thwart Maj-Gen Soleimani's plans to attack Americans in Iraq and elsewhere in the Middle East, although officials have not yet provided specific intelligence to back up that claim.

Last week, pro-Iranian protesters rioted at the US Embassy in Baghdad, trapping diplomats inside for two days and setting on fire some buildings on the compound's outer perimeter.

In several interviews earlier on Sunday, Secretary of State Mike Pompeo avoided directly answering whether the US would attack cultural sites in Iran. He said on ABC's This Week that the US would "behave lawfully" and "behave inside the system".

But the targeting of cultural sites is against international law and critics denounced Mr Trump for his statement.

"I think this is the president using puffery and trying to sound tough in a way that just reveals his ignorance," said Mr Scott R. Anderson, a former State Department lawyer during the Obama administration who is now a national security law expert at Columbia University and the Brookings Institution.

Mr Anderson, who was the legal adviser at the US Embassy in Baghdad in 2012 and 2013, said the Pentagon had long recognised that strikes should only include targets of what he described as military necessity.

"So you can't just start shooting anything you want as a hostage target, like a cultural site," Mr Anderson said on Sunday in an interview. He is also advising Mr Pete Buttigieg's Democratic presidential campaign but was not speaking on its behalf.

The US is a signatory to a 1954 international agreement to protect cultural property in armed conflict. Violating it with attacks on Iran's historical sites would represent a huge turnabout. The US was among the harshest critics of the Islamic State's destruction of antiquities in Mosul, Iraq and Palmyra, Syria, as well as the Taleban's obliteration of the Bamiyan Buddhas in Afghanistan in 2001.

"The US has taken a leadership role in the protection of antiquities from destruction and illicit trade, particularly in the Middle East," said Ms Deborah Lehr, chairman and founder of the Washington-based Antiquities Coalition.

"It would be a shame to see that global goodwill disappear by the intentional targeting and destruction of cultural sites."

The International Criminal Court convicted an Al-Qaeda-linked extremist of war crimes in 2016 for destroying historic and religious artifacts in Mali. But the US is not a party to the court, which is based in The Hague, the Netherlands.

In 2018, the Trump administration withdrew the US from Unesco, the cultural organisation of the United Nations that is known to travellers for its list of World Heritage Sites.

Beyond official condemnation from across the world, other signatories to the 1954 convention could refuse to be enlisted by the US for military actions against Iran, Mr Anderson said. That could include withholding intelligence or refusing to let US forces prepare for attacks on Iranian interests from bases in allied nations.

"There are real practical costs to this," Mr Anderson said.

By Sunday, under the hashtag #IranianCulturalSites, a Twitter campaign cropped up in the form of history buffs taking verbal aim at Mr Trump's threat.

Among the sites cited as irreplaceable treasures - not just for Iran, but also for antiquities preservation globally - was Persepolis, parts of which still stand.

Its ruins were among the first three Iranian sites to be placed on the Unesco list, in 1979. Built in 518 BC, the city was the capital of the Achaemenid Empire. It was conquered and looted by Alexander the Great in 330BC but remains "among the world's greatest archaeological sites" for its evidence of ancient architecture, urban planning and art, according to Unesco.

It is "of the last standing massive archaeological complexes from ancient Persia", tweeted one user, who identified himself as Sergio Beltran-Garcia and listed architecture as an interest. "The Iranians and their cultural institutions have done a fantastic job in protecting it."

Even some of the people closest to those taken hostage by Iran appeared to disdain Mr Trump's threat.

"Here's a thought, maybe ask ex-hostages if they want to each be assigned to represent a military target that could kill many civilians, supposedly in their honour," tweeted Ms Sulome Anderson, a journalist who is the daughter of Mr Terry Anderson, a former Associated Press bureau chief who was kidnapped by Iranian-backed Hezbollah in Lebanon in 1985 and held for six years.

She signed her post: "Sincerely, Daughter of an American ex-hostage of an Iran-backed group."