The Syrian president, Bashar al-Assad, has said Russian anti-aircraft missiles have arrived in Syria, Lebanese TV has reported – a claim likely to dramatically increase tensions in the region and which could provoke the Israelis to launch a strike against the weapons.

In an interview with the al-Manar channel, to be broadcast on Thursday, Assad reportedly confirmed that Moscow had begun to deliver the long-range S-300 air defence rockets. "Syria has received the first shipment … All our agreements with Russia will be implemented and parts of them have already been implemented," he said.

Assad also claimed that his forces were winning the battle on the ground and had scored "major victories" against the armed opposition. "The balance of power is now with the Syrian army," he told the TV channel, which is owned by the Shia militant group Hezbollah, whose fighters support the Assad regime.

The deal between the Kremlin and the Syrian government has been in train for some time. But the S-300 shipment, if confirmed, appears to be Russian retaliation for the European Union's controversial decision this week to lift an arms embargo on Syria. Britain, France and states in the region such as Turkey are now actively seeking to arm the country's moderate rebels.

There was confusion on Thursday in western capitals as to whether Assad's claim was true. US officials said they had no evidence that the S-300 shipment had arrived. One high-ranking Israeli official said: "We are trying to find out exactly what the situation is but currently we just don't know." Analysts said it was possible some elements of the S-300 system – made up of launchers, radar and a command-and-control vehicle – had turned up but doubted the system was yet operational.

Nonetheless, senior Israeli figures signalled the possibility that Israel could now launch a pre-emptive attack on Syria with a view to destroying the S-300s, which it sees as an existential threat. Earlier this week the defence minister, Moshe Ya'alon, said Israel could not afford to allow the new system to become fully functional.

The Israeli official added: "This move will certainly change the whole dynamic [of Israeli involvement in the Syrian conflict]. This is mostly as a result of the EU's reckless decision to lift the arms embargo. I don't know if the shipment of the missiles was a direct result of that decision, but it certainly gave the Russians a pretext to go ahead and do what they wanted to do in the first place. If they had any misgivings, doubts about the timing, the EU decision rid them of these."

Others went further and suggested there was a genuine prospect the Russian delivery could trigger a destructive Israeli-Syrian war. Earlier this month, Israeli warplanes struck targets near the capital, Damascus, reportedly wiping out Iranian missiles destined for Hezbollah. Syria did not respond. "I'm quite convinced that the more times we attack, there will be accumulative pressure on the [Syrian] regime to react," Shlomo Bron, a senior Israeli security figure, said. "It's difficult to know when and where this breaking point will be. But the likelihood of a very real flare-up between Israel and Syria is much higher than it used to be."

Major General Giora Eiland, a former Israeli national security adviser, added: "If we do do something soon after the transfer, we might have business not only with Syria but with the Russians. This is a real hot potato."

The Foreign Office described Russia's move as unhelpful. It said Damascus had proved itself "incapable of using its weapons systems proportionately or discriminately" and had fired lethal Scud missiles against its own cities, such as Aleppo. The focus should now be on the political process, the Foreign Office said.

But attempts to convene an international peace conference next month in Geneva appeared to be on the brink of failure after Syria's opposition announced it was pulling out. The fragmented National Coalition for Syrian Revolutionary and Opposition Forces said it would not participate in "meaningless" talks while the Assad government continued to massacre Syrian civilians. It also decried "Iran's malicious invasion of Syria" – a reference to Iran's active role in the conflict on the side of Assad. Tehran has dispatched Revolutionary Guards to train pro-government militias, and is a key sponsor of Hezbollah, which has vigorously supported the Syrian regime. US, Russian and UN officials had been due to meet in Geneva next Wednesday before the conference.

The developments on Thursday raise fears that the two-year Syrian struggle is morphing inexorably into a high-stakes regional war, as well as a cold-war-style proxy conflict pitting an implacable Russia against the west. Russian officials did not comment on Thursday on whether Assad's claims were true. Previously, however, the Kremlin said it had agreed to deliver the S-300s to Syria before the civil war erupted, and added that the S-300s could help reduce tensions by deterring what it called "foreign interference". The Washington Post on Thursday published a leaked document suggesting Moscow supplied the Assad regime with 20,000 Kalashnikov assault rifles and 20m rounds of ammunition in March alone.

Assad's comments on Thursday suggest the Syrian government has been emboldened by a series of recent victories over the rebels, and is now working on the assumption that it will defeat the insurgency. On Wednesday, Syria's foreign minister, Walid al-Moallem, told Lebanon's al-Mayadeen TV that Damascus would retaliate immediately if Israel struck Syria again. He also said there was no prospect of Assad stepping down before his current term expired in 2014, and signalled the president might run for office once more.

On the ground, Assad's forces are closing in on the strategic town of Qusair after taking control of the area following a bloody two-week battle with opposition forces.

The Syrian army seized the nearby Dabaa airbase on Wednesday – a major blow to the rebels in Qusair, an overwhelmingly Sunni town in the western part of the country that the opposition had controlled since early last year.

The town lies along a land corridor linking Assad's two key strongholds: the capital Damascus and his Alawite heartland along the Mediterranean coast. For the rebels the town has been vital in maintaining a supply line – of fighters, guns, ammunition and other supplies – to Lebanon, just six miles away. On Thursday the opposition National Council said more than 1,000 wounded people were stuck in Qusair, and encircled by regime forces.

"It is not reasonable, it is not logical that people and civilians are getting killed minute by minute while the international community carries on in a standstill," Khalid Saleh, a spokesman for the council, said.