Oren Dorell

USA TODAY

The United Nations said Tuesday the bombardment of Aleppo by Russia and Syria — which has killed more than 400 people and injured hundreds more over the last two weeks — may constitute a war crime and should be referred to the International Criminal Court.

The statement came as Russia announced it has deployed an advanced anti-aircraft missile system to Syria, and warned the U.S. against attacking its ally.

A referral to the International Criminal Court "would be more than justified given the rampant and deeply shocking impunity that has characterized the conflict and the magnitude of the crimes that have been committed, some of which may indeed amount to war crimes and crimes against humanity," Zeid Ra'ad al Hussein, the U.N.'s high commissioner for human rights, said in a statement.

The commissioner called for a change in U.N. rules that allow permanent members of the Security Council to veto referrals to the International Criminal Court. The permanent members are the United States, Russia, China, France and the United Kingdom.

Zeid warned Russia against using incendiary weapons in Aleppo, and said world leaders should “limit the use of the veto by the permanent members of the Security Council,” which would allow the U.N. body to refer the situation in Syria to the International Criminal Court.

A cease-fire brokered by the U.S. and Russia collapsed on Sept. 19 as fighting has continued in Syria.

Russia on Tuesday said it deployed its advanced S-300 anti-aircraft missile system to Syria, amid warnings against attacks on Syrian forces.

The Syrian Arab Republic received an S-300 anti-aircraft missile system,” said Igor Konashenkov, a spokesman for the Russian Defense Ministry, according to state-owned broadcaster RT. “This system is designed to ensure the safety of the naval base in [Syrian city of] Tartus and ships located in the coastal area” of Syria, he said.

On Oct. 1, Russian Foreign Ministry spokeswoman Maria Zakharova warned against “direct aggression” by the U.S. against Damascus and the Syrian army.

Such U.S. aggression “would cause a terrible tectonic shift not only in the country, but in the entire region,” Zakharova said.

Of the 1,400 killed or wounded since the ceasefire collapsed, 425 were dead, including 293 non-combatants, according to the monitoring group Syrian Observatory for Human Rights reported Tuesday. The London-based monitoring agency's numbers could not be independently verified.

The killings resulted from airstrikes conducted by Russia and the Syrian government, which targeted homes and hospitals in civilian areas, the observatory said. The onslaught against civilians is aimed at forcing them to flee to government-controlled parts of the city, the observatory said.

Dozens of people were killed Tuesday as Syrian rebels fought both the Islamic State and government forces loyal to Syrian President Bashar Assad, the observatory reported.

A three-way battle continued in Damascus province between opposition forces and the Islamic State, which launched an offensive in the area and killed three rebels in a suicide attack, the Observatory reported. In separate fighting, four government soldiers were captured by al-Qaeda’s Syrian franchise, formerly known as the Nusra Front, which is allied with some opposition forces. In Hama province, six government soldiers and an unknown number of opposition fighters were killed in a rebel offensive.

The U.S. on Wednesday will join talks to pursue a diplomatic solution to the Syria conflict, with representatives from the United Kingdom, France, Italy and Germany. Notably absent will be Russia.

The U.S suspended bilateral talks with Russia about Syria on Monday, citing that country’s ongoing airstrikes in Aleppo despite a cease-fire agreement.

U.S. suspends Syria ceasefire talks with Russia

The cease-fire, which began Sept. 12, was rocky from the start, but collapsed a week later when an airstrike that the U.S. blamed on Russia hit a United Nations aid convoy in Aleppo, killing 20 people.

“Russia and the Syrian regime have chosen to pursue a military course, inconsistent with the (cease-fire), as demonstrated by their intensified attacks against civilian areas, targeting of critical infrastructure such as hospitals, and preventing humanitarian aid from reaching civilians in need,” State Department spokesman John Kirby said in a statement.

The U.S. administration is not giving up on finding a political solution. But any further cease-fire arrangements with Russia would have to include an end to Russian attacks on "civilian populations" and the grounding of Syrian President Bashar Assad's air force, Mark Toner, a deputy spokesman for the State Department, told reporters Wednesday.

Officials from the U.S. and Russian militaries, which are both conducting airstrikes in Syria, will continue to consult with each other to ensure their air operations do not conflict, but the U.S. is now considering other options in Syria, deputy spokeswoman Elizabeth Trudeau said Monday.

President Obama’s National Security Council is considering various options to press Russia and the Syrian government to stop their offensives and return to the negotiating table. “Those conversations have been going on,” she said.