President Bashar al-Assad has warned that the fall of his regime or the breakup of Syria will unleash a "domino effect" that will fuel Middle East instability for years to come.

This is his sharpest warning yet about the potential fallout of his country's civil war on neighbouring states.

In Moscow, President Vladimir Putin said the Syrian conflict has become "a massacre".

Mr Putin said that it must be stopped through peace talks, and repeated the Kremlin's firm rejection of calls for Assad's ouster.

President Assad accused his neighbours of stoking the revolt against his rule and warned they would eventually pay a heavy price.

He said: "We are surrounded by countries that help terrorists and allow them to enter Syria," 

Mr Assad said: "Everybody knows that if the disturbances in Syria reach the point of the country's breakup, or terrorist forces control Syria ... then this will immediately spill over into neighbouring countries and there will be a domino effect that will reach countries across the Middle East." 

The Syrian regime is under growing pressure from an increasingly effective rebel force that has managed to pry away much of northern Syria and is making significant headway in the south.

Rebel forces have been capturing military bases and territory that could offer rebels a staging ground to attack the capital, Damascus, the seat of President Assad's power.

The rebel gains coincide with what Western and Arab officials say are US-backed training of opposition fighters in Jordan and an influx of foreign-funded weapons into the south.

The rebel advances have given the opposition momentum and put the government on the defensive in the 2-year-old conflict that the UN estimates has killed more than 70,000 people.

President Assad also lashed out at Turkey's prime minister, Recep Tayyip Erdogan, who was a close ally before the crisis began but has turned into one of his harshest critics.

Turkey has been one of the strongest backers of the Syrian opposition, providing it with logistical support and shelter.

"When the prime minister (Erdogan), or the government or officials get involved in shedding Syrian people's blood there is no place for bridges between me and them or the Syrian people that don't respect them," Mr Assad said.

The president criticised President Erdogan for reconciling with Israel after three years of cold relations, and accused the Turkish leader of "working in coalition with Israel to strike against Syria."

Mr Assad used the interview to quash rumors that he had been killed by one of his guards or that he has been in hiding.

"I am present in front of you and not in a shelter. These are mere rumours," he said.

The Syrian revolt started with largely peaceful protests in March 2011.

The revolt has morphed into a civil war with increasingly sectarian overtones.

Sunni Muslims dominate rebel ranks, while the Assad regime is composed mostly of Alawites, an offshoot Shiite group to which the president and his family belong.

Russia, a close Assad ally, has shielded Damascus from UN sanctions and largely stood by the regime.

Although Russia has also signalled that it is not tied to his remaining in power.

At the same time, it has refused to back calls for Assad to step down, and has instead pushed for talks with the opposition.

Speaking to the German ARD television in remarks released by the Kremlin on Friday, Putin repeated Moscow's firm rejection of calls for Assad's ouster.

"What is going on is a massacre, this is a disaster, a catastrophe," Putin said. "It has to be stopped."

He added, however, that "when they say that Assad is fighting against his own people, we need to remember that this is the armed part of the opposition."

Mr Putin said that negotiations between the government and the opposition are necessary to provide guarantees to all parties and prevent the country from sliding into turmoil, as befell Libya, Iraq and Yemen.

Inside Syria, rebel forces continued to make gains in the south, overrunning an army garrison that defends the main border crossing with Jordan after a weeklong siege, said the Britain-based Syrian Observatory for Human Rights.

Earlier in the day, activists said a barrage of rockets slammed into a contested district on the northeastern edge of Damascus.

At least five people were killed and others trapped under the rubble.

The attack on the capital's Barzeh neighborhood, where opposition fighters are known to operate, followed days of heavy fighting between the rebels and the military in the area.

Meanwhile, the Italian Foreign Ministry said four Italian journalists were kidnapped in northern Syria, the Italian news agency ANSA said. It provided no further details.