UN Secretary General Ban Ki-moon met here on Monday with Myanmar's President Thein Sein in his first visit to the country under a new reformist regime in a high-profile show of support for changes sweeping the former pariah state.  "I would like to extend a warm welcome from the people of Myanmar," President Thein Sein said as the pair met at his official residence in the capital Naypyidaw.  Ban offered international help for reform in the long-isolated state and will later become the first foreign dignitary to address its fledgling parliament.  The UN Chief is also scheduled to address Myanmar's Parliament later Monday.  Myanmar's quasi-civilian government embarked on a wave of political and economic liberalisation, Ban described Thein Sein, a former top general, as a "key driver" of the reforms and said he would urge Western powers to ease sanctions further.  The new government, comprising mostly former generals in the military junta that ruled for half a century, has stunned the world with the pace of its reforms in a way that Ban said was impressive but still a work in progress.  "We need to support Myanmar so it doesn't slide back down the scale," Ban told reporters in the capital, Naypyitaw, on Sunday, the first day of a three-day visit.