US dependence on the Or-ganisation of Petroleum Exporting Countries for its oil imports has risen to its highest level in 15 years, following Angola's arrival this week as the 12th member of the oil producers' group.

Angola's entry was agreed at the Opec meeting in Abuja, Nigeria, last month, and was formalised on Monday. The African country is the first new member to join the oil cartel since 1975.