The condemnation is the latest by human rights advocates against Philippine President Rodrigo Duterte, whose 13-month- old administration has triggered widespread alarm, notably over his deadly drug war.
"The Government of the Philippines must urgently address growing reports of human rights violations, including murder, threats against indigenous peoples and the summary execution of children," the panel of three experts said in a statement.
One of the experts cited is the UN's special rapporteur on extra-judicial killings, Agnes Callamard, who has previously faced harsh criticism from Duterte loyalists over an unofficial visit she made to the Philippines in May that the government claims was not authorised.
"All these cases must be investigated thoroughly and perpetrators should be brought to justice", added the statement, co-signed by Michel Forst, the UN expert on human rights defenders and Ms Maud de Boer-Buquicchio, the special rapporteur on the sale and sexual exploitation of children.
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While the statement did not specifically cite Duterte's drug war, that campaign has been a key element of what the experts called "a climate of prevailing violence" in the Philippines.
More than 2,000 other people have been killed in drug- related crimes, according to police data.
Rights groups say many of those victims have been killed by vigilante death squads linked to the government, and that Duterte may be overseeing a crime against humanity.