The United Nations human rights chief Michelle Bachelet has been authorised to collect and preserve evidence of alleged war crimes during Sri Lanka's long civil war, which ended in 2009.
The UN believes 80,000-100,000 people died in the 26-year conflict with the Tamil Tiger rebel group.
A resolution passed by the UN Human Rights Council accused Sri Lanka of "obstructing accountability".
Sri Lanka described the resolution as "unhelpful" and "divisive".
The council's resolution also called for trials of suspects in foreign countries.
Both the government and the Tamil Tiger rebels are accused of atrocities during the conflict.
Speaking to the BBC on Tuesday, Ms Bachelet said: "I welcome the Human Rights Council's decision to continue its monitoring of human rights in Sri Lanka and pursue accountability for past crimes. I salute the courage and commitment of victims from all communities in Sri Lanka in their pursuit of truth and justice."
She said she hoped Sri Lanka would "change course from its current policies" and "ensure full protection of minorities, human rights defenders and the media".
In the war's final few months, the Sri Lankan army pinned the rebels into a strip of north-eastern coast, where the government says the rebels kept thousands of civilians as human shields, exacerbating the death toll.
Yasmin Sooka, a rights lawyer involved in a civil prosecution against Sri Lanka's current President Gotabaya Rajapaksa for alleged war crimes, said the resolution was "hugely significant for victims".
"It really is a recognition that at the domestic level, processes have failed and there is no real hope that victims will access justice," she told the Reuters news agency.
The push to pass the resolution was led by the UK. It passed by 22 votes to 11, with China and Pakistan among those voting against. There were 14 abstentions, including that of neighbouring India.
Ahead of the vote, the British ambassador Julian Braithwaite said: "We bring this resolution in the hope that it will help advance the human rights of all communities, including Buddhist, Christian, Muslim, Sinhala or Tamil.
"And more importantly that it will help prevent the recurrence of past grave violations of human rights which the High Commissioner warned this council about in such clear terms."
The success of the resolution gives Ms Bachelet's office new staff, powers and a $2.8 million budget to look at Sri Lanka's war with a view to future prosecutions.
At a press conference in Sri Lanka's capital Colombo, the nation's Foreign Minister Dinesh Gunewardena said the resolution lacked authority as more nations voted against or abstained than in favour.
"The resolution was brought by countries supported by Western powers that want to dominate the Global South," he said.
The UN Human Rights Council also expressed alarm at the human rights situation in Sri Lanka today, including a "deepening and accelerating militarisation of civilian government functions", "the erosion of the independence of the judiciary", and "increased marginalisation" of Tamil and Muslim minorities.
Anbarasan Ethirajan, World Service South Asia Regional Editor
Despite weeks of intense behind-the-scenes diplomatic campaigning, the UN Human Rights Council resolution has gone against the Sri Lankan government.
The new resolution gives more powers to the UN human rights chief to ensure accountability for the alleged war crimes during the conflict with the Tamil Tiger rebels which ended in 2009, and the evidence gathered can be used for any future prosecutions.
Sri Lanka is worried that in the long term it may lead to travel and other restrictions on some of the military and civilian officials involved in the war. The government has been defiant, arguing that the resolution cannot be implemented without the consent of the country concerned.
But some Tamil community leaders believe that the UN resolution did not go far enough to impose tougher sanctions on Sri Lanka. For them, their wait to find answers about their relatives missing in the war continues. They want accountability for the thousands of deaths.
The resolution may not satisfy all the parties concerned, but it's a clear reminder from the international community that the alleged abuses in the Sri Lankan conflict will not be forgotten or ignored.