U.N. war crimes investigators said Thursday they have asked for an urgent meeting with President Bashar Assad to press for a firsthand look at his war-torn nation.

Since last year an independent commission appointed by the U.N. Human Rights Council has unsuccessfully sought unrestricted access to Syria.

Sergio Pinheiro, the Brazilian professor and diplomat who heads the four-person panel, has only been allowed into Damascus for a weekend last summer to meet with government officials and some families affected by the violence.

Instead, the panel has gathered nearly all of its evidence of suspected war crimes and other abuses from more than 1,100 interviews with victims and others who have fled Syria.

Activists say more than 35,000 people have been killed since the start of the anti-Assad uprising that began in March 2011.

"We'll again seek our access to Syria and we decided to send a letter to President Assad calling for a meeting," Pinheiro told reporters at the U.N.'s European headquarters in Geneva, after a first meeting of the newly reconstituted panel that now includes Carla del Ponte, a former U.N. war crimes prosecutor. "We expect that he will receive us in Damascus."

Pinheiro said the panel had a "duty" to demand access to Syria. "We intend to go there without conditionalities," he added.

Del Ponte, who also served as Swiss attorney general, spent eight years with the International Criminal Tribunal for the former Yugoslavia pursuing former Yugoslav nationalist leader Slobodan Milosevic, who died in 2006 while on trial for his crackdown on ethnic Albanian separatists in Kosovo, a province of Serbia.

Asked whether she sees similarities with Syria, del Ponte replied yes.

"The similarity of both is that we are handling the same crimes: crimes against humanity and war crimes, for sure," she said.

The council, which is the U.N.'s top human rights body, appointed the panel to keep investigating at least until its meeting next March.

In its latest report in August, the investigators said Assad's regime and pro-government shabiha militia were directly responsible for the killing of more than 100 civilians in Houla in late May and numerous other murders, unlawful killings, acts of torture, rape and other sexual violence and indiscriminate attacks on civilians .

The panel also concluded that anti-government groups committed war crimes, including murder, extrajudicial killings and torture on a lesser frequency and scale.