Trump threatens to strike Iran's bridges and electric power plants

US President Donald Trump has warned late on Thursday about striking and destroying bridges and electric power plants in Iran in his latest threat to hit the country's infrastructure.

The US military "hasn't even started destroying what's left in Iran. Bridges next, then Electric Power Plants," Trump wrote on social media.

His post said that Iran's leadership "knows what has to be done, and has to be done, FAST!"

Trump, who has previously offered shifting timelines and objectives for the war, said in a televised speech on Wednesday that the war could escalate if ​Iran did not give in to Washington's terms, with strikes on its energy and oil ​infrastructure possible.

Dozens of international law experts in the US signed an ​open letter released earlier on Thursday saying that US strikes on Iran may amount to war crimes.

The 1949 Geneva Conventions on humanitarian conduct in war prohibit attacks on sites considered essential for ‌civilians.

The Geneva Conventions and additional protocols say that parties involved in military conflict must distinguish between "civilian objects and military objectives", and that attacks on civilian objects are forbidden.

"We are going to hit them extremely hard over the next two ​to three weeks. We are going to bring them back to the ​Stone Ages, ⁠where they belong," Trump said in his Wednesday address.

While he said Washington was nearing the completion of its goals in Iran, Trump did not lay out a timeline to end the war.

The war began on February 28 ⁠when the ​US and Israel attacked Iran. Tehran responded by launching its ​own attacks on Israel and Gulf states with US bases. Joint US-Israeli strikes in Iran and Israeli attacks in Lebanon ​have killed thousands and displaced millions.

The war has also raised oil prices and shaken global markets. Trump's mixed messages thus far have done little to ease the concerns over his country's biggest military attacks since the 2003 invasion of Iraq.