UNITED NATIONS (AP) — The head of the independent U.N. commission on Syrian war crimes said Friday that commanders of the Islamic State extremist group are "good candidates" to be put on a confidential list of alleged perpetrators.

Brazilian diplomat and scholar Paulo Sergio Pinheiro said the al-Qaida breakaway group, which now controls a swath of north and eastern Syria, has carried out public executions, crucifixions and other "gross human rights violations."

Pinheiro singled out the Sunni militant group when he provided details of the categories of people on the list, which is kept under lock and key by U.N. High Commissioner for Human Rights Navi Pillay, to U.N. Security Council members and a group of journalists for the first time.

He said the list includes people who are "criminally responsible for hostage-taking, torture and executions;" heads of Syrian intelligence branches and detention facilities where detainees are tortured; military commanders who target civilians; airports where bombing attacks are launched; and armed groups and individuals involved in "attacking and forcibly displacing civilians."

The Syrian government's command structure has barely changed during the more than three-year war, he said, but in the case of most opposition groups it is difficult to identify commanders — except in the case of the Islamic State group.

Pinheiro said The Independent International Commission of Inquiry for Syria, which he heads, has determined that the Islamic State group, which began with many foreign fighters, has attracted Syrian fighters from other armed opposition groups.

The commission, which is appointed by the U.N. Human Rights Council, estimates there are between 10,000 and 15,000 foreigners fighting in the numerous groups opposed to President Bashar Assad's government, he said.

Pinheiro refused to disclose any names or the number of people on the confidential list of suspects the commission is compiling for possible future prosecution, but he said additions continue to be made.

Members of the commission had urged the Security Council to refer the conflict in Syria to the International Criminal Court, but Russia and China, supporters of the Syrian government, vetoed a resolution that could have brought the key perpetrators of war crimes to justice.

Pinheiro said another possibility is "a hybrid court" with U.N. and national participation, like the tribunals for Sierra Leone, Lebanon and Cambodia, but that would require council approval as well.

Nonetheless, Pinheiro said, he is certain that those responsible for war crimes in Syria will eventually face justice.