Ex-president Mahinda Rajapaksa's brother Gotabaya Rajapaksa denounced the United Nations human rights chief's visit to Sri Lanka as a "big joke", as the former regime stepped up opposition to a UN-backed war-crimes probe.

Former president Rajapaksa and his brother, ex-defence secretary Gotabhaya Rajapaksa, signed a petition this morning against the probe into allegations of thousands of civilian deaths during the final months of Sri Lanka's separatist war.

Zeid Ra'ad Al Hussein arrived on Saturday (Feb 6) for a four-day visit to gauge the island's progress in investigating war-time atrocities, before he delivers an assessment to the UN Human Rights Council in March.

Gotabhaya Rajapaksa accused authorities of arranging for Zeid to meet only sympathisers of Tamil rebels, who were crushed by government troops in 2009, AFP reported.

"He can't come here for a day and expect to understand the situation. He is only meeting one side," Gotabhaya, who was defence secretary during the war's finale, told reporters in Colombo. "It is a big joke."

Flanked by the ex-president, Gotabhaya Rajapaksa repeated the former regime's longstanding position that no war crimes were committed by government troops in the final push.

After defeating Rajapaksa at presidential elections a year ago, his successor Maithripala Sirisena agreed to investigate allegations troops killed up to 40,000 Tamil civilians in the final months.

Zeid on Sunday (Feb 7) visited the former war zones of Jaffna and Trincomalee in the island's north and northeast.

Meanwhile, he told local Tamil leaders on the Jaffna peninsula, which saw some of the worst fighting, that there should not be a general amnesty, but a swift legal process to deal with rebel detainees.

"As a general principle it is not acceptable to grant amnesties to those convicted of the most serious crimes - war crimes or crimes against humanity," a spokesman for the rights chief told AFP.

But the UN would welcome a release of those against whom there was insufficient evidence.

More than 200 suspected Tamil separatists remain in prison, many without charge.

Tamil political and civil society groups have long demanded their unconditional release, though the government has refused a blanket amnesty.

(With inputs from AFP)