NAYPYIDAW  - The first US ambassador to Myanmar in over two decades will arrive to take up his post on Wednesday, US officials said, as dramatic reforms spur greater engagement with the longtime army-run nation. Derek Mitchell, a veteran US policymaker on Asia, will travel to the purpose-built capital Naypyidaw to meet Myanmar's President Thein Sein, according to a statement from the US embassy in Yangon. Mitchell was nominated to the role by President Barack Obama, who has pursued a policy of greater engagement with Myanmar as the Southeast Asian nation emerges from decades of junta rule, which ended last year. The US has eased some of its strict sanctions against the country as a reward for a series of reforms including holding landmark by-elections that saw Aung San Suu Kyi and other members of her party elected to parliament. Mitchell arrives in Naypyidaw days after Suu Kyi -- whose democracy struggle saw her locked up for 15 of the last 23 years by the generals -- made her debut in parliament, lending legitimacy to a legislature that remains dominated by the army and its political allies. Washington withdrew its ambassador to Myanmar after a crackdown on a democracy uprising in 1988 and elections won by Suu Kyi's democracy party two years later that were never recognised by the junta. But a recent slew of positive changes from Thein Sein's quasi-civilian government, which took power last year, have surprised the West and driven hopes of a democratic future for the country.