Four months of Russian air strikes in Syria are taking their toll on rebel forces, strengthening the hand of a defiant President Bashar al - Assad as the United Nations struggles to get peace talks off the ground. Insurgents in the west are being hit harder, while in eastern and central parts of the country, Islamic State is also under military pressure and is cutting fighters' pay as its oil-smuggling operations are hit by plunging prices. Rebel groups are reporting intensified air strikes and ground assaults in areas of western Syria that are of greatest importance to Assad. The government last week made one of its most significant gains since the start of the Russian intervention, capturing the town of Salma in Latakia province. While recent gains do not appear to mark a tipping point in the conflict, with rebels fighting back and regaining positions in some places, insurgents describe high levels of attrition on the front lines of western Syria. Officials close to Damascus say sealing the northwestern border with Turkey is the priority. A Syrian military source said rebel supply lines from Turkey, which backs the insurgents, were under pressure from Russian and Syrian air strikes. The course of battle underlines the uphill struggle facing U.N. envoy Staffan de Mistura as he seeks to launch peace talks. Even with U.S. and Russian endorsement, a new peace process seems detached from the realities of a five-year-old war that may not yet be ready for peacemaking. "Most opposition-held areas turned to defense because of the huge mobilization by Russians troops and the use of a large number of planes with unlimited munitions," said Jamil al-Saleh, commander of a rebel Free Syrian Army (FSA) group. While playing down the importance of government gains, Saleh said military aid from the rebels' foreign backers - including Saudi Arabia and Turkey - was not enough to confront offensives that are also backed on the ground by Iran. "These are among the difficulties facing the FSA on the ground especially since the aerial bombing is affecting some headquarters, equipment, cars and personnel and the aid given is little compared to the ferocious attack," he told Reuters. Saudi Arabia's support for the opposition has yet to be translated into the kind of heavier weapons the rebels are seeking, notably anti-aircraft missiles. The military source, speaking on condition of anonymity, said rebels were suffering from the destruction of their weapons depots, made possible by good intelligence. Their appeals for more support showed they had "lost a lot of field capacities", the source said.