Ammon News - COLOMBO (Al Jazeera) - The UN's top rights official is to begin a fact-finding mission to Sri Lanka after the government dropped public hostility towards her and promised access to former war zones.

Navi Pillay, who has previously been accused by Colombo of overstepping her mandate, will on Sunday start a week-long mission that will include talks with President Mahinda Rajapakse and trips to the former war zones in the north and east of the island.

The government's U-turn comes as Canada leads calls for a boycott of a Commonwealth summit scheduled to take place in the Sri Lankan capital later this year.

Sri Lanka has resisted pressure from the UN and Western nations for an investigation into allegations that up to 40,000 civilians were killed in the final months of its separatist war, which ended in 2009.

"She has not accepted what we have done (to improve the rights situation)," Sri Lanka's human rights envoy to the UN, Mahinda Samarasinghe, told reporters in Colombo last week.

"So we are showing her what we have done and we are also allowing her to visit anywhere and meet anyone."

A military offensive crushed Tamil Tiger rebels who at the height of their power controlled a third of Sri Lanka's territory. Rajapakse has since been dogged by claims of indiscriminate killing of ethnic Tamils.

Tamil groups are banking on Pillay's first visit to Sri Lanka to revive calls for a war crimes probe.

Her visit follows two resolutions by the UN Human Rights Council (UNHRC) in as many years demanding Colombo hold an independent investigation into "credible allegations" that troops shelled hospitals and refugee camps, and executed surrendering rebels.

The government insists that its troops did not kill civilians and has slammed the UNHRC for "ill-timed and unwarranted" resolutions. Pro-government activists have led demonstrations outside UN offices in Colombo, accusing Pillay of being a US stooge.

Until recently, the government declared much of the former northern war zone off limits to foreign journalists, aid workers and UN staff.