Turkey says doubled tariffs on Turkish steel and aluminium announced by US President Donald Trump are against the rules of the World Trade Organisation (WTO).

"Turkey expects other member countries to abide by international rules," the country's trade ministry said in a statement.

It said it would support steel and aluminium exporters on all international platforms, and the US remained an important trade partner.

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Mr Trump intensified his spat with Turkey by imposing the higher tariffs, putting unprecedented economic pressure on a NATO ally and deepening turmoil in Turkish financial markets.

The new duties on Turkey are double the level Mr Trump imposed in March on steel and aluminium imports from a range of countries.

In a tweet, he said he was imposing duties of 20 per cent on Turkish aluminium and 50 per cent on steel.

The White House said Mr Trump had authorised them under a section of US trade law that allows for tariffs on national security grounds.

'Their currency slides rapidly downward'

The Turkish lira dropped as much as 18 per cent at one point after Mr Trump's announcement — the biggest one-day fall since a 2001 financial crisis in the country.

This year, it lost more than 40 per cent.

"Their currency, the Turkish lira, slides rapidly downward against our very strong dollar!" Mr Trump said.

"Our relations with Turkey are not good at this time!"

Reverberations spread through global markets, with European stock markets especially hit as investors took fright over banks' exposure to Turkey. US stocks were also rattled.

The lira has long been falling on worries about Mr Erdogan's influence over monetary policy and worsening relations with the United States.

Turkish President Tayyip Erdogan told Turks to sell their gold and dollars to support the crumbling lira.

No breakthrough from talks

Without naming countries, Mr Erdogan said supporters of a failed military coup two years ago, which Ankara said was organised by a US-based Muslim cleric, were attacking Turkey in new ways since his re-election two months ago.

Turkey wants the US to extradite cleric Fethullah Gulen, a Pennsylvania-based cleric who Turkish authorities say masterminded the coup attempt in which 250 people were killed. Gulen has denied the allegation.

Mr Erdogan has linked Gulen's fate to that of American Christian pastor Andrew Brunson, who is on trial in Turkey on terrorism charges for allegedly supporting a group that Ankara blames for the failed coup. He has denied the charges.

American Christian pastor Andrew Brunson is on trial in Turkey on terrorism charges. ( Reuters: Depo Photos )

His cause resonates with Mr Trump's Christian conservative supporters.

Turkish officials held talks in Washington this week, but there was no breakthrough.

Although Mr Erdogan struck a defiant tone, his foreign ministry called for diplomacy and dialogue to solve problems with the US.

"We implore President Trump to return to the negotiating table," Trade Minister Ruhsar Pekcan said.

ABC/Reuters