Emerging market stocks fell to two-week lows on Friday, with steelmakers taking a pounding after President Donald Trump said he would impose hefty tariffs on US imports of steel and aluminium, reviving fears of a trade war.

Trump said the duties of 25% on steel and 10% on aluminium would be formally announced next week, risking retaliation from key trading partners. Among emerging market exporters, Brazil, South Korea and China are considered the most exposed to the new tariffs.

The news triggered a broad-based sell-off across developed and emerging markets, with MSCI's emerging stocks index down 0.7% in a fourth straight day of losses and set to end the week down 2.6%.

"I assume there will be some retaliatory measures," said Per Hammarlund, chief emerging markets strategist at SEB.

"Countries might go through the WTO (World Trade Organisation) first to see if they can get a ruling in their favour, but it's unlikely to make a big difference as Trump is unlikely to abide by any of their rulings. So it will be up to the individual countries to decide on their response."

A possible course would be for countries to target specific US products that would have an impact on a par with the steel and aluminium tariffs, he suggested.

South Korea - the third-largest steel exporter to the United States after Canada and Brazil - said it would keep talking to US officials until Washington's plans are finalised.

Shares in Seoul fell over 1% to a three-week low with Dongkuk Steel tumbling over 5%, while Posco and Hyundai Steel dropped some 3%.

Hong Kong shares slid 1.5% while Chinese mainland shares fell 0.8%, with steelmaker Baoshan Iron & Steel and Aluminium Corp of China suffering.

Although China accounts for only 2% of US steel imports, its massive industrial expansion has helped produce a global glut of steel that has driven down prices.

Trade tensions between the world's top two economies have already hotted up in recent weeks: China launched an anti-dumping and anti-subsidy investigation of imports of US sorghum after Trump slapped steep tariffs on imports of solar panels and washing machines.

The sell-off spread widely through emerging markets. South African stocks fell 1%, dragged down by steelmaker ArcelorMittal tumbling 5.5%.

Moscow shares fell 0.3%, with Russian steel producer NLMK shedding 1.9%, and coal and steel producer Mechel 1.3%. Polish stocks slipped 0.8% to their lowest since July 2017.

Earlier, Brazil's industry ministry said it would consider taking action on its own over the tariffs, or in concert with other countries. Two of its largest steelmakers, Cia Siderurgica Nacional and Usiminas, closed down more than 4% on Thursday.

However, the weaker dollar helped shelter emerging currencies. Russia's rouble, South Korea's won and China's yuan firmed around 0.2%, though all three currencies were on track for weekly losses.

The rand was treading water after data showed South Africa's budget deficit widened year-on-year in January. However, the currency was on track for a 2.6% weekly fall, its biggest weekly drop since October.

In emerging Europe, the Polish zloty remained under pressure, extending losses against the euro to trade at three-week lows. European Union lawmakers are supporting the European Commission's plans to take unprecedented punitive steps against Poland over reforms of its judiciary and state media that they say threaten the rule of law in the ex-communist country.