EKATERINBURG, Russia- Russian President Vladimir Putin said Tuesday that Moscow had not yet delivered its sophisticated S-300 missiles to the Syrian regime for fear this would upset the delicate balance of power in the volatile region.

His comments came as the United Nations reported there were “reasonable grounds” to conclude that chemical weapons had been used by both Syrian President Bashar al-Assad’s forces and the rebels battling to overthrow him.

Fighting meanwhile raged on in the key Syrian city of Qusayr as both sides battled for control of a city that provides a direct link between the regime’s seat of power in Damascus and the Mediterranean Sea coast.

Putin -who has shielded the Assad regime from three attempts to impose UN sanctions during the 26-month conflict -said Moscow had already signed a contract for delivery of the powerful S-300 surface-to-air missile systems.

The weapon could theoretically shoot down Israeli jets in the region and harm any Western military support missions. But he appeared to back down from Moscow’s earlier insistence that the missiles would be a “stabilising factor” in the Middle East, saying no shipments had been made so as not to upset the region’s delicate balance of power.

“The contract was signed several years ago. It has not been realised yet,” Putin said at a joint press conference with European Union leaders in the Ural Mountains city of Yekaterinburg. “We do not want to upset the balance in the region.”

But he warned the West and Arab states allied to the opposition that any attempt to resolve the crisis through outside force was “doomed to failure.”

A crisis that has already cost more than 94,000 lives has threatened in recent weeks to turn even more dire with reports that chemical weapons -albeit in small amounts -had been used by both sides in the brutal war.

UN investigators in Geneva added to those suspicions by reporting they had grounds to believe chemical weapons had been used by both sides. “Allegations have been received concerning the use of chemical weapons by both parties,” said the Commission of Enquiry on Syria in a report to the UN Human Rights Council.

There are “reasonable grounds to believe that chemical agents have been used as weapons,” added the investigators. It was the first time the commission -tasked with probing human rights violations in Syria since 2011 -added the suspected use of chemical agents to its long list of war crimes committed in the conflict-torn country.

Putin himself referred to the war crimes allegations by saying that widely circulated footage of a rebel eating the body parts of a slain Syrian soldier had threatened to derail a proposed Syrian peace conference that Moscow and Washington are working to organise.

“I hope that such participants of the negotiations do not appear” at the proposed Geneva talks, Putin said. “Otherwise, it will be hard for me to ensure the safety of the Russian participants. It would be hard to work with such people.”

Uncertainty in recent days over whether Russia had in fact delivered S-300s to Syria had further strained Moscow’s already tense relations with regional governments and the West.

Israeli Prime Minister Benjamin Netanyahu paid a special visit to Russia last month to convince Moscow not to make the shipments.

US Secretary of State John Kerry also voiced concern. The Syrian crisis and the European Union’s decision to lift its arms embargo on Assad’s foes dominated the regular Russia-EU summit.

European Council President Herman Van Rompuy said Brussels expressed its full support for the joint Russia-US initiative for a peace conference involving both Assad’s camp and the opposition to be held as soon as possible. “What is happening in Syria today is a stain on humankind’s conscience,” said European Commission head Jose Manuel Barroso.

The latest fighting on the ground saw a missile strike near Syria’s biggest city Aleppo kill 26 people and government warplanes pound Qusayr.

The Syrian Observatory for Human Rights said there were numerous dead on both sides but gave no other details. The Observatory also said shellfire near the Russian embassy in Damascus had killed a civilian and wounded a member of the security forces.

US-based group Human Rights Watch meanwhile said its mission to Aleppo had concluded that the bodies of 147 men pulled out of a local river between January and March were “probably” executed in government-controlled areas of the northern city.