(Reuters) - Mexico, the United States and Canada have signed off on amending the North American Free Trade Agreement (NAFTA) to liberalize rules of origin for a host of products, the Canadian prime minister's Web site said.

Products covered include natural gas, pharmaceuticals, cosmetics, rubber, metals, industrial and electrical machinery and precision instruments, the site said, adding they account for $166 billion in annual trilateral trade.

(Reporting by Ana Isabel Martinez and Veronica Gomez)