Worries about the dollar overshadowed the opening of Opec's head of states summit on Saturday, after a warning by a senior Saudi minister that the US currency could "collapse" if the oil cartel mentioned its weakness in the leaders' closing declaration.

In a landmark summit, leaders of the Organisation of the Petroleum Exporting Countries are meeting in Riyadh, Saudi Arabia, were divided over how they should respond to the weakness of the US dollar, which has fallen 16 per cent this year against a basket of leading currencies.