Inbursa, the bank owned by Carlos Slim, the Mexican billionaire, has bought up to $150m in Citigroup shares over the past week as the US financial group’s stock plunged in value.

Inbursa, Mexico’s sixth largest bank, began buying the stock as prices fell on Wednesday last week, and continued the next day when they punctured the $5 mark. The bank’s brokerage arm is believed to have bought as many as 29m Citigroup shares for about $150m.