US President Donald Trump faced further outrage over the weekend after tweeting the US could attack 52 Iranian sites, "some at very high level & important to Iran & the Iranian culture", if Tehran attacks a US person or target.He later added: "Should Iran strike any US person or target the US will quickly and fully strike back and perhaps in a disproportionate manner."The tweets, sent in the aftermath of the killing of Iranian General Qassem Soleimani in Iraq, sparked widespread condemnation, with many rights experts and organisations pointing out that targeting cultural sites with military action is considered a war crime under international law. The outrage only prompted Trump to initially double down, however."They're allowed to kill our people. They're allowed to torture and maim our people. They're allowed to use roadside bombs and blow up our people. And we're not allowed to touch their cultural sites. It doesn't work that way," he told reporters on Sunday. READ | Soleimani killing: Iran's Zarif vows response to US 'act of war'Many experts agree that if Trump were to follow through on some of the threats, including the use of disproportionate force, it would violate international law.The principle of proportionality is one of the key principles of international humanitarian law.The principle prohibits attacks against military objectives which are "expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated," according to the Red Cross. Using disproportionate force amounts to a war crime.Luciano Zaccara, professor in Gulf Politics at Qatar University argued that the US has already taken disproportionate force by killing Soleimani, "one of the highest authorities within the Iranian political-military establishment".READ | Trump tweets Iran will 'never have a nuclear weapon'According to Ilias Bantekas, professor of law at Hamad bin Khalifa University in Doha, the tweet in which Trump threatens to act "disproportionately" is a violation of Article 2 (4) of the UN Charter which states that all members of the UN shall "refrain from the threat or use of force against the territorial integrity or political independence of any state"."Even if an action were deemed to have been lawful as a matter of self-defence, its disproportionate character would make it unlawful."Kevin John Heller agreed, saying that while there would be "nothing inherently wrong for Trump to say - in his own unique way - that the US would defend itself against an armed conflict", the fact that he threatened disproportionate force "is in violation of Article 2(4)" of the UN charter.These Media Posts will serve as notification to the United States Congress that should Iran strike any U.S. person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner. Such legal notice is not required, but is given nevertheless!— Donald J. Trump (@realDonaldTrump) January 5, 2020But Charles Blanchard, a partner at Arnold Porter law firm and a former general counsel in the US Army, disagreed."The tweet is muddled enough that it is difficult to consider it as an official threat by the US," Blanchard said.Michael Pregent, a former US intelligence officer and senior Middle East analyst at the Hudson Institute, also said the tweets were not in violation of international law."It's a warning to Iran that if they attack American interests, Iran will be held accountable and the US will target things that are of interest to Iran," Pregent told Al Jazeera.ALSO READ | Trump's Middle Eastern aggression, state capture in Nzimande and SACP's cross-hairsBantekas added that "the novelty here is that unilateral action is undertaken through Twitter and that the threat of force emanates not through official channels such as the State Department but from a private tweet."He said that "a tweet is a private action, but when used by a president to effectively threaten armed force, it transformed into a state act."