ANKARA (Reuters) – Turkish Foreign Minister Mevlut Cavusoglu said on Tuesday the Syrian army was striking moderate opposition forces in Idlib province and this was undermining efforts to reach a political solution to the war in Syria.

Turkey has been fiercely opposed to Syrian President Bashar al-Assad during his country’s six-year-old civil war but has recently been working with his allies Russia and Iran for a political resolution to the conflict.

The three countries agreed last year to establish a “de-escalation zone” in the opposition-held Idlib province and surrounding region, which borders Turkey, but Syrian forces have since launched an offensive in the area.

“Regime forces are hitting the moderate opposition with the excuse that they are fighting Nusra (Islamist militants). This attitude scuppers the political solution process,” state-run Anadolu news agency quoted Cavusoglu as telling reporters.

“The groups who will come together in Sochi should not do this,” he said, referring to the Russian city where a Syrian congress of national dialogue is set to be held at the end of this month.

The main rebel force in the northwestern province of Idlib is Tahrir al-Sham, spearheaded by the former al Qaeda affiliate in Syria that was known as Nusra Front.

A Turkish security source said Ankara was watching developments in northern Syria closely but added that the areas that the Syrian army had recaptured lay “mostly outside the de-escalation zones”.

Turkey says it has deployed troops to observation points in northern Idlib province, about 60 km (40 miles) north of the Syrian army offensive.

While Ankara has toned down its criticism of Assad since it started cooperating with Russia, President Tayyip Erdogan last month called Assad a terrorist and said it was impossible for Syrian peace efforts to continue with him.