Last week's announcement have led to widespread concerns about a potential trade war
Senior Republican Paul Ryan has publicly urged Donald Trump to not proceed with proposed steel & aluminum tariffs, saying he is 'extremely worried' about the potential consequences.
Last week President Trump announced plans for a 25% tariff on steel imports and a 10% levy on aluminium, despite reports that his own advisers had urged him not to do so.
The US President said the move was in response to decades of "disgraceful" unfair trade policies.
He argued: "We must not let our country, companies and workers be taken advantage of any longer."
The move sparked immediate fears of a trade war amid indications that other countries could impose retaliatory trade barriers of their own.
President Trump himself took to Twitter to claim "trade wars are good".
When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win. Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy!— Donald J. Trump (@realDonaldTrump) March 2, 2018
The announcement saw world stock markets plunge, while many US commentators - including from President Trump's own party - raised concerns about potential higher costs for US manufacturers.
On Monday, House Speaker Paul Ryan became the most senior Republican critic of the plan.
In a statement quoted by US media outlets, Mr Ryan's spokesperson said: "We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan.
"The new tax reform law has boosted the economy and we certainly don’t want to jeopardise those gains."
President Trump, meanwhile, has continued defending his plan.
In a tweet on Sunday, he argued: "We are on the losing side of almost all trade deals. Our friends and enemies have taken advantage of the U.S. for many years. Our Steel and Aluminum industries are dead. Sorry, it’s time for a change!"
The World Trade Organisation Director-General Roberto Azevêdo has urged countries to avoid an 'escalation of trade barriers'.
Addressing WTO members today, he argued: "We cannot ignore this risk and I urge all parties to consider and reflect on this situation very carefully. Once we start down this path, it will be very difficult to reverse direction.
"An eye for an eye will leave us all blind and the world in deep recession."
The threats of a trade war come as the US, Canada and Mexico continue their efforts to renegotiate the terms of the North American Free Trade Agreement (NAFTA).
On Monday, President Trump took to Twitter in an apparent effort to increase pressure on the USA's two neighbours.
We have large trade deficits with Mexico and Canada. NAFTA, which is under renegotiation right now, has been a bad deal for U.S.A. Massive relocation of companies & jobs. Tariffs on Steel and Aluminum will only come off if new & fair NAFTA agreement is signed. Also, Canada must..— Donald J. Trump (@realDonaldTrump) March 5, 2018
...treat our farmers much better. Highly restrictive. Mexico must do much more on stopping drugs from pouring into the U.S. They have not done what needs to be done. Millions of people addicted and dying.— Donald J. Trump (@realDonaldTrump) March 5, 2018