US President Donald Trump has threatened further tariffs and export restrictions against countries that he says are targeting American tech firms.

"Digital Taxes, Digital Services Legislation, and Digital Markets Regulations are all designed to harm, or discriminate against, American Technology," Trump wrote on his Truth Social platform.

His comments appear aimed at European countries and others who have created laws regulating the online world.

The European Union for example has rules like the Digital Markets Act and the Digital Services Act covering competition and content moderation.

The UK also has a tax on digital companies, although Trump did not name specific countries or regions.

But he said that unless what Washington deemed as "discriminatory actions" were removed, he would "impose substantial additional tariffs" on countries' products and institute export restrictions on US tech and chips.

"America, and American Technology Companies, are neither the 'piggy bank' nor the 'doormat' of the World any longer," Trump wrote.

In June, Trump called off trade talks with Canada in retaliation against Ottawa's planned digital services tax, which would have applied to US multinational companies such as Alphabet, Amazon and Meta that provide services to Canadians.

Shortly after, Canada said it would rescind the taxes impacting US tech firms.

Since returning to the presidency in January, Trump has imposed tariffs on allies and competitors alike, with varying rates for different countries' products.

But sectors he has singled out for separate action – such as steel, aluminium and autos – are spared from these countrywide levels.

Instead, they have been hit with sector-specific duties.

Trump has wielded tariffs as a means to pursue various goals.

A doubling of US tariffs on Indian products is set to take effect this week over its purchase of Russian oil – a key revenue source in Moscow's war in Ukraine. (AFP)