(CNN) -- The Tamil National Alliance, Sri Lanka's largest Tamil political party, on Thursday called for an international inquiry into a U.N. failure to protect civilians during the final stages of the country's bloody 26-year civil war.
The request comes a day after the United Nations admitted that it didn't protect hundreds of thousands of civilians, mostly ethnic Tamils, trapped in areas of heavy shelling in Sri Lanka, a teardrop-shaped island nation off India's southern coast.
"The internal inquiry report that has now come to light says the U.N. failed civilians by leaving the area," M.A. Sumanthiran, the Tamil National Alliance parliamentarian, told CNN.
Sumanthiran was referring to the withdrawal of the U.N.'s international staff from north Sri Lanka's Wanni area. In late 2008, the government could no longer guarantee the safety of U.N. staff there because of intense fighting between its forces and the Liberation Tigers of Tamil Eelam, or LTTE, an armed separatist group commonly referred to as the Tamil Tigers.
As the U.N. prepared to leave, local civilians pleaded with it to stay, fearing the government would bomb areas where there were no international witnesses. In the report released Wednesday, the United Nations questioned its decision to move its staff "because of a government safety warning when government forces themselves represented the dominant threat to staff."
The U.N. also acknowledged that it had, on occasion, "omitted to explicitly mention government responsibility for violations of international law" so as to not hinder its humanitarian access in the country.
The internal review published accounts in which U.N. officials emphasized grave responsibility for civilian deaths with the LTTE but only "raised concerns" with the Sri Lankan government.
The government of Sri Lanka declared victory in 2009 in its decades-long battle with the Tamil Tiger rebels, who had waged war for an independent state for minority Tamils in Sri Lanka since July 1983. As many as 70,000 people were killed in the conflict.
Caught in the crossfire year after year were civilians, thousands of whom were displaced and hundreds killed.
In the war's final stage, which lasted from September 2008 to May 2009, the Sri Lankan army advanced into an area of northern Sri Lanka known as the Vanni, where about 330,000 people were trapped by fighting.
A U.N. report in 2011 said the government used "large-scale and widespread shelling" that left large numbers of civilians dead.
In its report this week, the U.N. admitted to withholding the large number of civilian casualties it had recorded from diplomatic corps in the country.
Though the United Nations says it made taciturn criticism of the government so that it could negotiate pauses in fighting to allow civilians to escape and for it to gain access to internally displaced people, the efforts were made in vain because, as Wednesday's report said, the U.N. was never in a position to reach those displaced people.
The number of civilian deaths and injuries are unknown to this day, and U.N. figures greatly differ from those in reports from Sri Lanka's government and other nongovernmental organizations.
"The report concludes that the United Nations system failed to meet its responsibilities," United Nations Secretary-General Ban Ki-moon said in a statement Thursday.
The U.N. report concludes by stating: "The UN's failure to adequately respond to events like those that occurred in Sri Lanka should not happen again. When confronted by similar situations, the UN must be able to meet a much higher standard in fulfilling its protection and humanitarian responsibilities."
However, Sumanthiran says there is more work to be done before moving on.
"That does not mean that it is a lesson for the future for the United Nations. The United Nations should revisit the events that took place during the final stages of the war in May 2009, where both the Tamil Tiger rebels and the security forces are accused of committing war crimes," he said.
"If the U.N. has made a mistake, they must pursue that course of action."
The Sri Lankan government had no immediate official response to the United Nations report.
CNN's Iqbal Athas in Colombo, Sri Lanka, contributed to this report.