(Reuters) - The world could slip back into a Cold War over Syria and the sprawling Arab country could break up into two or three warring parts, with unforeseeable consequences for the Middle East, a senior Israeli military commander said.

"Support for (Syrian President Bashar) Assad from Russia and China is taking us back to the Cold War," he said this week, on condition of anonymity. "The world is not a one-man show."

A regional proxy war is already under way in Syria, he said, with direct, daily, on-the-ground support for Assad from his allies in Iran and Lebanon´s heavily-armed Hezbollah movement.

"There can be real chaos. It can take years," he said.

The 15-month-old conflict in Syria has grown into a full-scale civil war, the U.N. peacekeeping chief said on Tuesday.

Hundreds of civilians, rebels and members of Assad´s army and security forces have been killed since a ceasefire deal brokered two months ago was meant to halt the bloodshed.

Russia and China backed the United Nations plan to send in military observers to check on adherence to the truce, but have refused to consider Western calls for a U.N. Security mandate that would authorize force, including military intervention.

The West has repeatedly said it has no plan to intervene, but has not ruled it out.

"In Syria, a proxy war is under way with Iran supplying arms to its Alawite client and Turkey actively arming the opposition," says Can Kasapoglu, a Turkish analyst who is currently a visiting fellow at Israel´s Begin-Sadat think tank.

The rebel Free Syrian Army is getting support from Sunni states Turkey, Saudi Arabia and Qatar, all allies of Washington.

Recent video of spectacularly successful attacks destroying Syrian tanks suggests the rebels may have obtained modern anti-tank weapons more powerful than rocket-propelled grenades.

Washington says Russia may be sending attack helicopters to its ally Syria. Claims by Moscow that its arms transfers to Syria are unrelated to the conflict are "patently untrue," U.S. Secretary of State Hillary Clinton said on Tuesday.

Russia´s foreign minister on Wednesday defended his country´s sale of arms to Syria and accused the United States of supplying rebels with weapons to fight against the government.

"We are not violating any international law in performing these contracts," said Sergei Lavrov at a news conference in Tehran shown on Iranian state television.

"They (the United States) are providing arms and weapons to the Syrian opposition that can be used in fighting against the Damascus government," he said, speaking through an interpreter.

PROXY WARS

The exchange was reminiscent of Cold War rhetoric when proxy wars were frequent. The superpowers, who could not risk a direct nuclear- armed confrontation between each other, battled for hegemony by involvement on warring sides in third countries.

From 1945 to the collapse of Soviet communism in 1989 there were proxy wars in Greece, Korea, Vietnam, Lebanon Afghanistan, Angola, Mozambique, Cuba, El Salvador and Nicaragua.

In the post-Cold War world, America was the only superpower but spheres of influence were heeded.

Moscow did not take on NATO when its former Yugoslav ally Serbia was bombed by the Western alliance in 1999 over the civil war in Kosovo.

In the former Soviet republic of Georgia, Russia was able to successfully back its secessionist allies militarily without triggering a war with the United States.

In Libya last year, however, Moscow was stung by NATO´s military intervention under a U.N. mandate it believed had been stretched beyond the limits it had agreed to.

Israel sees the Syrian civil war becoming part of the struggle for dominance in the Arab world between Sunni and Shi´ite Muslims. "Shia are only 20 percent of Muslims in the world but have taken the lead away from the Sunnis," he said.

FLASHPOINT

"Assad has seen the death of Gaddafi in Libya and the fate of Mubarak in Egypt and he understands he has no choice. He knows his Alawite minority will be slaughtered," the officer said. "We all know the end of the story. We just don´t know the chapters."

The question is who might grab the lead in "this Sykes-Picot country", he said, referring to Syria´s creation by colonial powers Britain and France after the First World War, on what look like arbitrary geographical lines that disregard tribal and ethnic distinctions.

"Who will replace Assad? Will it be all those doctors in Europe (Syrian National Council in exile) or will it be al Qaeda?" said the officer, adding U.S. ally Saudi Arabia was very concerned.

"It is not a nation state like Iran and Egypt are. It can become two or three states."

The risks of a regional war were clear, he said, as key U.S. Middle East ally Israel faces the possibility of its sworn enemy Iran becoming a nuclear-armed state and contemplates whether military action will be needed in the end to stop it.

Israel has to be prepared, he said.

"You don´t know what will trigger it, but everything is ready for a big, big fire. You don´t know who will strike the match." (Reporting By Douglas Hamilton; Editing by Sophie Hares) (© Thomson Reuters 2012. 06/13/12)