SIEM REAP, Cambodia — Secretary of State Hillary Rodham Clinton declared Myanmar open to American investment Friday, introducing the reformist president to leaders of some of the biggest US corporations and prodding him to do more to expand democracy in his long-reclusive country.

Following the Obama administration’s recent loosening of sanctions against Myanmar, Clinton met President Thein Sein for an hour in the Cambodian city of Siem Reap, praising him for several prodemocracy reforms.