Mike Waltz has defended Donald Trump's latest threat to attack civilian infrastructure in Iran by saying the US has a 'long history of taking down bridges'. 

Trump wrote on Truth Social that there would be 'no more Mr Nice Guy', while pledging to 'knock out every single power plant and every single bridge' in Iran if they refuse the next peace deal, as the ceasefire is set to expire in three days.

The comment, which came on the eve of peace talks between the US and Iran in Islamabad, Pakistan, sparked a backlash from several Democrat politicians, who have accused the president of unleashing 'devastation' on the Iranian people. 

US Ambassador to the UN Waltz leapt to Trump's defense during a fiery exchange with NBC News reporter Kristen Welker on Meet the Press on Sunday morning. 

'I just want to get ahead of this ridiculous and frankly irresponsible narrative that this would somehow constitute a "war crime," that we've heard from Democratic politicians, the media, and others in the international community,' he said.

'That is wrong. It's irresponsible. We have a long history of taking down bridges, power plants, and other infrastructure that is powering Iran's military. 

'And they have a long history of conflating civilian and military assets including hiding weapons caches, rockets and military hardware in schools, hospitals and other civilian neighborhoods, which is a war crime on Iran's part.'

The interview grew increasingly heated, as Waltz swerved several key questions about the conflict in Iran and Welker grew visibly irate. 

Welker attempted to interrupt Waltz several times as he continued to slam Iran. 

'Are you saying the United States is okay targeting civilians, the people of Iran?' she eventually interjected. 

'In the laws of land warfare and the rules of engagement, any type of infrastructure that is being comingled, that is being used to move military hardware is absolutely a legitimate target as our power plants, the grid, and other things that are being used to power Iran's war economy,' Waltz said. 

'The president will move up an escalation ladder responsibly... until Iran comes to the table and really lives up to a responsible deal that the Vice President has put on the table,' he added.

Democratic Congressman Ro Khanna was among those who pushed back against the president's most recent threats to bomb civilian infrastructure in Iran. 

'They said they want to escalate to de-escalate. They've escalated to devastation. I mean, you have the pope lecturing America about possible war crimes,' Khanna told ABC's This Week on Sunday. 

'You have the president, as you pointed out, threatening to destroy all power plants. I didn't think we would ever get to that point.'

A US delegation has been sent to Pakistan for negotiations in the Middle Eastern conflict, which escalated February 28 when the US began combat operations in Iran. 

When asked whether the president would extend the ceasefire agreement if a peace deal could not be reached before it's expiry on Wednesday, Waltz said: 'Everything's on the table. 

'The president has made it very clear that he is prepared to escalate in order to de-escalate this conflict.' 

Waltz also said that no country should be able to 'close down international waterways' in reference to Iran - despite Trump threatening to do the same thing on April 12.

'You can't have a situation where a party to a conflict regardless of whether you agree with it or not, can close down international waterways, whether it's the Strait of Gibraltar, the Bearing Strait, the Strait of Malacca,' he said.  

'And this is why we've been so active at the United Nations. And this is why, you know, President Trump has said this cannot and will not stand.'

Trump said he was blockading the Strait of Hormuz on after peace talks with Iran collapsed when JD Vance returned from Pakistan empty handed on April 12. 

'Effective immediately, the United States Navy, the Finest in the World, will begin the process of BLOCKADING any and all Ships trying to enter, or ​leave, the Strait of Hormuz,' Trump wrote on Truth Social at the time. 

However, two days later, the US military said it would enforce a blockade against of all vessels attempting to leave or enter Iranian ports or the Iranian coastline, including the Gulf of Oman and the Arabian Sea. 

Trump appeared to climb down from his initial threat, writing on April 17: 'THE STRAIT OF HORMUZ IS COMPLETELY OPEN AND READY FOR BUSINESS AND FULL PASSAGE, BUT THE NAVAL BLOCKADE WILL REMAIN IN FULL FORCE AND EFFECT AS IT PERTAINS TO IRAN, ONLY.' 

Vice President JD Vance will lead the next round of peace talks in Islamabad, Pakistan, ahead of the ceasefire expiration date on Wednesday, according to ABC News.

Trump told Fox News that US Special Envoys Jared Kushner and Steve Witkoff would also be part of the American delegation. 

'If Iran does not sign this deal, the whole country is getting blown up,' Trump said, according to Fox. 

Iran has doubled down on its pledge to restrict ships passing through the Strait of Hormuz as long as the US blockade of Iranian ports remains in place, as mediators scrambled to extend the ceasefire.

The dueling blockades have complicated Pakistani-led mediation attempts and raised questions about whether the two-week truce can be extended. 

'It is impossible for others to pass through the Strait of Hormuz while we cannot,' Iranian parliamentary Speaker Mohammad Bagher Qalibaf said in an interview aired on state television late Saturday.

Qalibaf, who is Iran's chief negotiator in talks with the United States, slammed the US blockade as a 'naive decision made out of ignorance'.