"I am not here to criticise. I've come to raise human rights concerns," she told reporters as she began her first meeting with local UN staff in Colombo.
Pillay, who is on her first visit to Sri Lanka, said she has framed herself "within the human rights standards set by the whole world inlcuding the government of Sri Lanka".
Her visit came after the Sri Lankan government, already under pressure from Canada for a boycott of a Commonwealth summit here in November, dropped public hostility towards her and promised access to former war zones.
A South African national of Indian Tamil origin, Pillay will meet President Mahinda Rajapaksa and travel to former war zones in the northern and eastern provinces during her visit.
"When I go back I would be reporting to the Human Rights Council," Pillay said.
The UN official will provide a first hand assessment on Sri Lanka at the September session of the UN Human Rights Council. She will provide a full formal report in March 2014, in accordance with the resolution adopted by the Council earlier this year.
Pillay said she had delayed her visit despite a government invitation until Sri Lanka presented the report of its Lesson Learnt and Reconciliation Commission (LLRC).
Sri Lankan government's nationalist allies charge that Pillay was part of a sinister Western conspiracy to undermine the country.
As many as 40,000 people were reportedly killed in the last months of the conflict between Sri Lankan troops and the LTTE rebels fighting for an independent homeland. The war ended in 2009.
