(Adds more details from Libya, bylines) By Ahmed Elumami and Peter Cooney TRIPOLI/WASHINGTON, June 14 (Reuters) - A veteran Islamist militant blamed for masterminding an Algerian gasfield attack and running smuggling routes across **rth Africa, has been killed in a U.S. air strike inside Libya, Libya's recognized government said on Sunday. The government said the strike had killed Mokhtar Belmokhtar, an Algerian militant who became a major figure in insurgencies across **rth Africa and the Sahel and had earned the nickname the "Uncatchable" from the French military.