GENEVA: The United Nations' top human rights official called on Wednesday for urgent international action to protect civilians in Syria, saying she was appalled by the Syrian government's military onslaught on the city of Homs.

Navi Pillay, High Commissioner for Human Rights, also implicitly criticized Russia and China for vetoing an Arab and Western move in the U.N. Security Council to get Syria's President Bashar Assad to quit.

"I am appalled by the Syrian government's wilful assault on the city of Homs, and its use of artillery and other heavy weaponry in what appear to be indiscriminate attacks on civilian areas in the city," Pillay said in a statement.

Anti-Assad activists in Syria say that, after an overnight artillery bombardment, government forces are thrusting into rebel-held areas of Homs with tanks as troops fired rockets and mortars, leaving many dead.

Pillay, a former South African high court judge said it was extremely urgent "for the international community to cut through the politics and take effective action to protect the civilian population."

She said "the virtual carte blanche" granted to Syria by the veto of the U.N. resolution "betrays the spirit and the word" of a 2005 global accord on collective action to protect civilians when their government was failing to do so.

Her statement came as both Russia and China continued to defend their stance.

In Moscow, Russian Foreign Minister Sergei Lavrov said it was "not really the international community's business" to try to determine the outcome of a national dialogue between government and opposition in Syria.

China said its veto was based on the U.N. Charter and principles and a Chinese foreign ministry spokesman said that Beijing would make "unremitting efforts for the peaceful resolution of the Syrian crisis".

Pillay said over the past 11 months of a "brutal government crackdown on largely peaceful protests in Syria," thousands of people had been killed, detained, tortured and abducted.

Officials in her Geneva office say the deaths are now so widespread around the country that they can no longer issue reliable estimates. Last December she put the figure at more than 5,000.

"All evidence points to the involvement of the Syrian army and security forces in the perpetration of most of these crimes," said Pillay, who is also a former judge in the International Criminal Court.

"In the light of their nature and scale, they may constitute crimes against humanity, punishable under international law.

"Those in command should remember that there is no statute of limitations for serious international crimes and that there will be a sustained effort, for as long as it takes, to bring justice to all those who have been victims of the gross and systematic crimes taking place in Syria today."