A new U.N. investigation into allegations of war crimes in Sri Lanka will intensify international pressure on its government and could probe the purported responsibility of senior Sri Lankan officials with U.S. ties.

As many as 40,000 civilians may have died amid government shelling in the final five months of the conflict in 2009, according to a previous U.N. report. Government forces have also been accused of executing ethnic Tamil rebel leaders who tried to surrender.

U.N. High Commissioner for Human Rights Navi Pillay announced Wednesday a heavyweight panel including a Nobel laureate and a former judge on a U.N.-backed tribunal trying former Cambodian leaders for genocide and war crimes, to advise a 10-month investigation by her office.

The government denies its forces targeted civilians or committed serious abuses.