The agency was finally given permission to see the Rohingya after demanding for weeks to get access to them.
"They were in good condition,'' Kitty McKinsey, a spokeswoman for UNHCR, told The Associated Press.
"It's a big step forward that we have gotten access to them. We're now getting good co-operation from the Thai government to solve this issue.''
The officials were granted access to 12 young people, aged 14 to 17, from a group of 78 Rohingya who were found by the Thai navy on Monday night, McKinsey said.
A Thai court convicted the adult migrants detained with the minors of illegal entry on Wednesday, raising concerns they could be deported.
Meanwhile, the Myanmar government denied that the Rohingya who arrived in Thailand came from Myanmar, although a report on Friday in state-controlled media quoted the government as saying it would "take necessary measures and deal with the matter".
The Rohingya are a Muslim minority group who face persecution in Myanmar, whose military government does not recognise them as citizens.