The move, the company said, was necessary to avoid US sanctions on companies doing business with Iran.

ThyssenKrupp has about 27,000 employees in the US, and brings in about 8bn euros (�5.7bn; $9.3bn) a year from its contracts there.

In the wake of the 406m-euro buyback, ThyssenKrupp shares fell 2% in early trading in Frankfurt, having slid 6% in late trading on Monday.

The move was seen by investors as counterproductive, at a time when the company is trying to sell assets and reduce debt.

Longtime shareholders

The shares in question - 16.9 million of them - have been in Iranian hands for almost three decades, since well before the deposing of the US-backed Shah in 1979.

Since then, contact between the US and Iran has been limited, with sanctions applied to companies doing business there.

The US has now put its foot down about the Iranian shareholding, and ThyssenKrupp said that the company faced "serious, imminent harm" unless IFIC, the holding company to whom the shares belonged, dropped its stake from 7.79% to about 5%.

Buying the shares back - even at a stiff premium - meant that "threatened restrictions, based on relevant US legislation, relating to the unrestricted market access of ThyssenKrupp subsidiaries in the USA will be avoided, averting serious economic damage to the Group's business in the USA".