The British Museum said Friday it had returned to Iraq a collection of 156 cuneiform tablets believed to have been looted following the U.S.-led invasion of the country.

The items mostly dated from between 2100 BC and 1800 BC, the London museum said in a statement.

They were impounded by customs officials at a freight company near London Heathrow Airport in 2011.

The tablets are mostly economic documents but also include letters, legal and school texts and a mathematical document.

Many of them come from Irisagrig, an area that was heavily looted in the aftermath of the war.

The tablets were handed over to the Iraqi ambassador Saleh Altamimi and will be sent on to the Iraq Museum.

"The protection of Iraqi heritage is the responsibility of the international society which we hope to continue for future generations," Altamimi said in the statement.