Taking aim at China, Donald Trump's administration has launched an investigation into whether foreign-made steel poses a threat to US national security.

Launching the probe on Thursday (20 April), Commerce Secretary Wilbur Ross said China has increased its steel exports to the US despite promises to reduce them, adding that "China now commands 26% of the American market".

A memo from Trump under the Trade Expansion Act of 1962 – which permits serving US presidents to restrict trade if they determine it is harmful to US security – has already been inked, triggering the formal probe.

It would require Secretary Ross' department to report its conclusions to the White House within the next 270 days.

However, the scope of the Trump probe is not yet clear. It is not known whether any recommended restrictions or possible tariffs will fall only on China, or whether other nations also be the target of punitive action.

At present, the US imports steel from over 90 countries, including Canada, Mexico, Russia, Brazil, Turkey and Germany. According to the Associated Press, since signing the memo, President Trump has said the investigation into steel imports has "nothing to do with China" specifically, but he failed to mention which countries the probe would be looking at.