Ban Ki-moon, the UN secretary-general, has arrived in Myanmar for talks with Aung San Suu Kyi, the country's main opposition leader, and the reformist government amid a flurry of top-level visitors to the long-isolated nation.
Ban, who told reporters last week that Myanmar's transition had reached "a critical moment", is due to meet the Nobel laureate for the first time and speak in the country's fledgling parliament during his three-day trip.
The UN chief was met by senior local officials when he arrived in Yangon on a special flight on Sunday, UN information officer Aye Win said.
Top European Union diplomat Catherine Ashton is also in Myanmar following the recent suspension of the bloc's sanctions against the country as it seeks to reward political changes in the country.
Ashton on Saturday opened a new EU office in Yangon that will mostly oversee the management of aid programmes, but also have a political role, in a first step towards establishing a full diplomatic mission.
Myanmar, which languished for decades under repressive military rule, has seen a thaw in its relations with the international community since a controversial 2010 election brought a civilian government to power - albeit one with close links to the military.
The EU has responded to what it said were "historic changes" by suspending for one year a wide range of trade, economic and individual sanctions, although it left intact an arms embargo.
The UN secretary-general, who welcomed the EU moves and said he would discuss ways the UN could help the country, is due to travel to Naypyidaw, the capital, late on Sunday for talks with President Thein Sein on Monday.
He is then due to give an address to the country's parliament, the first by a visiting foreign dignitary.
The landmark speech would have been witnessed by Suu Kyi, who won a parliamentary seat in April 1 by-elections, but the democracy campaigner has shunned the legislature in a dispute over the swearing-in oath.
Suu Kyi's National League for Democracy has said it will not swear to "safeguard" an army-created constitution, in the first sign of tension with the government since the by-elections.
Thein Sein, a former army general, has ushered through a broad range of changes since coming to power last year, including welcoming Suu Kyi's party into the political mainstream and freeing political prisoners.
Ban is now set to meet Suu Kyi in Yangon on Tuesday.
A steady stream of foreign dignitaries, including British Prime Minister David Cameron and US Secretary of State Hillary Clinton, have visited Myanmar since the quasi-civilian government took power last year.
Canada also recently suspended most sanctions and Japan waived $3.7bn of Myanmar's debt.
But the US on Wednesday ruled out an immediate end to its main sanctions on Myanmar, saying it wanted to preserve leverage to push its leaders on an end to ethnic violence and other key issues.