In a rare interview, the Syrian president says a divided opposition could not uphold a peace deal and that he has no intention of stepping down

Syria's embattled leader Bashar al-Assad has used a rare interview – carried out amid the sound of artillery fire resounding through his presidential palace in Damascus – to warn the United States and Russia that their efforts to bring about talks will do little to halt the civil war laying waste to his country, and that he has no intention of stepping down.

In an exclusive interview for the Argentinian newspaper Clarín, shared with the Observer, Assad says he welcomes attempts at dialogue, but believes that western states are looking for ways to fuel the violence, rather than stop it, and are seeking to topple his regime regardless of the toll.

Moscow and Washington have been in dispute over the anti-Assad uprising since it began in March 2011 but are now trying to find common ground to quell the bloodshed and destruction as its effects continue to reverberate across the region. If successful, there are hopes talks could take place at the end of this month and lead to a multilateral summit attended by key protagonists.

Assad, speaking to Clarín's reporter Marcelo Cantelmi from the library of his palace, said that a continuing lack of unity between the myriad rebel groups meant that opposition leaders would be unable to implement any ceasefire measures agreed at a summit, such as surrendering arms. "They are not a single entity," he said. "They are different groups and bands, not dozens but hundreds. They are a mixture and each group has its local leader. And who can unify thousands of people? We can't discuss a timetable with a party if we don't know who they are."

Asked about the possibility of stepping down, he said: "I don't know whether [US secretary of state] John Kerry or anyone else has received a mandate from the Syrian people to decide whether someone should stay or go. Any decision about reforms in Syria will come from Syria and neither the US nor any other state can intervene. In any case, to resign would be to flee."

Attempts to consolidate a cohesive opposition force which is committed to Syria continuing as a pluralistic state have largely been unsuccessful. The war is now into its third year, sectarian positions are hardening and regional stakeholders are being drawn ever deeper into a conflict that threatens to also consume them. Assad again blamed Saudi Arabia, Qatar and Turkey for driving the insurgency, insisting that ending such support for the opposition must be a priority if the summit goes ahead. "There cannot be a unilateral solution in Syria; two parties are needed at least. In practice, the opposition forces are linked to foreign countries and cannot make a decision for themselves. They are one and the same, and it is they who announced that they don't want a dialogue with the Syrian state, most recently last week. Believing that a political conference will stop terrorism on the ground is unreal."

The Free Syrian Army remains nominally the umbrella rebel military group, but its power has been diminished by the rise of regional warlords and opportunists – and the creeping ascendancy of al-Qaida linked groups, which are now at the vanguard on numerous fronts. With central authority disintegrating, Syria is descending into an ungovernable domain of warlords, fiefdoms and militias, some of whom are fighting not for nationalistic aims but as part of a global jihad in the name of fundamentalist Islamist doctrine.

On both sides of the war, faith in the international community to bring about a solution has been evaporating rapidly. And in the opposition-held north of the country, there was growing frustration on Saturday at what is perceived as a disconnect between faltering global diplomacy and searing on-the-ground reality. "This is a fight to the death for the Sunnis," said Abu Hamza, a commander of a Free Syrian Army-linked brigade in Idlib province. "The regime has fired at least 200 ballistic missiles into the north against civilian areas. And the world wonders why we attack their villages? They are trying to eradicate us. We must get to them first."

Sectarianism, for so long a subcurrent in the Syrian conflict, is now a driving force for substantial elements on both the regime and opposition sides. A series of web videos posted in recent weeks chronicling atrocities committed by both sides reveals the growing depth of enmity and the willingness to lay claim to crimes that in the early months of the war would have been subject to interminable dispute. Assad denied credible reports that fighters from Lebanon's Hezbollah and the Iranian Revolutionary Guards had travelled to Syria to fight alongside his regime, but acknowledged that some members of both groups had been in the country.

"We do not have fighters from outside Syria," he said. "There are people here from Hezbollah and Iran, but they have been coming and going in Syria since long before the crisis." He again denied his regime had used chemical weapons, a claim regularly made by rebel groups and partly supported by western officials. He suggested that the use of such weapons could be used as a pretext to directly intervene in the crisis.

"It is probable that the issue would be used," he said. "The west lies and falsifies evidence to engineer wars, it is a habit of theirs. Of course, any war against Syria would not be easy, it wouldn't be a simple excursion.

"[Intervention] is a clear probability, especially after we've managed to beat back armed groups in many areas of Syria. Then these countries sent Israel to do this to raise the morale of the terrorist groups. We expect that an intervention will occur at some point, although it may be limited in nature."

He also rejected claims that his troops had used excessive force. "How does one define excessive force? How can one decide whether excessive force has been used or not? What is the formula to be applied?

The debate is not about the extent of the force used or the type of weapon … the issue really centres on the nature and extent of the terrorism we have suffered, and thus, what is a proper response."

Of the recent Israeli attacks, he accused Israel of doing the bidding of rebel groups, which he alleged had in turn bombed a Syrian military radar site, which allowed the Israeli jets to carry out their attack."Israel is directly supporting the terrorist groups in two ways, firstly it gives them logistical support and it also tells them what sites to attack and how to attack them. For example, they attacked a radar station that is part of our anti-aircraft defenses, which can detect any plane coming from overseas, especially from Israel."