The reformist government of the formerly junta-run nation committed to ending the recruitment and use of children in its "tatmadaw" army in a June 2012 pact with the UN.
A total of 646 children have since been released by the army, the UN said in a statement released Thursday.
"We welcome the official discharge of the children, including those who were accused of desertion -- and for not punishing them," said Renata Lok-Dessallien, UN resident coordinator in Myanmar, adding they should not have been in the army in the first place.
There are no verifiable figures on how many children currently serve in Myanmar's huge military, which has faced a raft of accusations over rights abuses, including the forced recruitment of children to work as porters or even human mine detectors.
The most recent batch of 51 discharged brings the total number of children released from the army this year to 93.
The UN statement said that all the young people released were under 18 at the time the pact was signed in June 2012.
As well as the army, the UN says at least seven rebel groups in Myanmar are known to recruit child soldiers.
In March, the country's quasi-civilian government secured a lauded draft national ceasefire deal with 16 rebel groups to end decades of fighting.
But this still needs to be agreed upon by rebel groups who are currently holding talks in Law Khee Lah, in eastern Karen state, on whether or not to sign a full ceasefire with the government.
The talks began Tuesday and were expected to last until June 6.
Myanmar is keen to achieve a ceasefire before landmark elections slated for November that are viewed as a key test of its transition towards democracy.
But in recent months the army has been locked in conflict with ethnic Chinese rebels in the Kokang region of the country's remote northern Shan state and faced outbreaks of fighting in northernmost Kachin.
Decades of iron-fisted junta rule and years of bloody conflict in Myanmar's borderlands have left a legacy of deep distrust of the military, which has long been accused of committing abuses with impunity.
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