The United Nations has a secret list of top Syrian officials who could face investigation for crimes against humanity carried out by security forces in their crackdown against an anti-government uprising, a panel of UN human rights experts said.

The UN experts indicated that the list goes as high as President Bashar Assad.

Thousands of Syrians have died in the violence since March and the panel, citing what it called a reliable source, said at least 500 children are among the dead.

"A reliable body of evidence exists that, consistent with other verified circumstances, provides reasonable grounds to believe that particular individuals, including commanding officers and officials at the highest levels of government, bear responsibility for crimes against humanity and other gross human rights violations," said the report by the UN-appointed Independent International Commission of Inquiry on Syria.

It added: "The commission has deposited with the United Nations High Commissioner for Human Rights a sealed envelope containing the names of these people, which might assist future credible investigations by competent authorities."

It does not say who these investigating authorities might be, but the UN's top human rights official has previously called for Syria to be referred to the International Criminal Court in The Hague.

Members of the 47-nation UN Human Rights Council are expected to hold a special meeting on Syria in Geneva next week, at which the panel's report will be presented. The panel, led by Brazilian professor Paulo Sergio Pinheiro, said its list also identifies some armed opposition cells thought to have committed gross abuses.

International pressure has been building on Assad's government to halt its violent suppression of the opposition. Earlier this week the International Committee of the Red Cross called for temporary ceasefires so it could reach those trapped and wounded in the worst-affected areas.

But human rights groups say the violence is only increasing, with dozens dying every day from government shelling of cities like Homs, a rebel stronghold.

The UN panel was denied entry to Syria by the government, which accused it of ignoring official information and exceeding its mandate. The panel instead gathered much of its information from sources outside the country, including human rights activists and Syrian army defectors.