Syrian President Bashar al-Assad said Turkish military and logistical support was the main factor that helped insurgents to seize the northwestern city of Idlib from government control last month.
Idlib, a short drive from the Turkish border, is only the second provincial capital to fall to insurgents in the four-year-long civil war. It was captured by an alliance of Islamist groups including al Qaeda's Syrian arm, the Nusra Front.
"Any war weakens any army, not matter how strong, no matter how modern," Assad said in an interview with Swedish newspaper Expressen, published on Friday.
In the fall of Idlib, "the main factor was the huge support that came through Turkey; logistic support, and military support, and of course financial support that came through Saudi Arabia and Qatar."
Turkish foreign ministry spokesman Tanju Bilgic, asked to comment by Reuters, said: "Claims that armed forces coming from Turkey have participated in the Idlib offensive do not reflect the truth. This is out of the question. These are baseless allegations orginated by the Syrian regime which should not be taken seriously."
The Syrian conflict is estimated to have killed around 220,000 people. Assad has lost control over much of the north and east while trying to shore up his control over the main population centres in the west, with the help of allies including Iran and the Lebanese group Hezbollah.
Starting next month, the UN envoy for Syria Staffan de Mistura is planning to consult Syrian protagonists and interested states on a new round of peace talks. These have consistently failed to make progress since the war erupted.
Asked about the initiative, Assad said the Syrian crisis had been complicated by external intervention.
Referring to states that are hostile to him, including Turkey, he said de Mistura was aware that if "he couldn't convince these countries to stop supporting the terrorists and let the Syrians solve their problem, he will not succeed".
The United States wants to see Assad gone from power and has shunned the idea of partnering with him against Islamic State, which has taken over large parts of Syria. In a series of interviews with Western media, he has repeatedly pressed his case that the jihadist groups in Syria pose a threat to Western states.
"Syria is a fault line," Assad said. "When you mess with this fault line you will have the echoes and repercussions in different areas, not only in our area, even in Europe."