ISTANBUL (Reuters) - Turkey has designated the insurgent group Tahrir al-Sham as a terrorist organization, according to a presidential decision published on Friday, as Damascus prepares for a military assault in northwest Syria where the group holds sway.

The notice in the Official Gazette matches a decision by the United Nations in June to add Tahrir al-Sham to the list of people and organizations whose assets are to be frozen because of links to militant groups al Qaeda and Islamic State.

It comes before an expected attack by the Syrian army, backed by Russia, on the northwestern Syrian region of Idlib which is home to nearly 3 million people on the border with Turkey.

Tahrir al-Sham, which includes the al Qaeda-linked group formerly known as Nusra Front, is the most powerful jihadist alliance in Idlib, the last major rebel-controlled enclave outside Syrian President Bashar al-Assad’s control.

Russia said on Friday the Syrian government had every right to chase terrorists out of Idlib, adding that talks were underway to set up humanitarian corridors there.

Turkish Foreign Minister Mevlut Cavusoglu said last week it would be disastrous to seek a military solution in Idlib, even though there were militants there.

Ankara, which has a small military presence in Idlib, has warned an assault could unleash a fresh wave of refugees.

“It is important for all of us to neutralize these radical groups,” he said. “But we have to distinguish the civilians from the terrorist groups.”

U.N. Syria envoy Staffan de Mistura said on Thursday there was a high concentration of foreign fighters in Idlib, including an estimated 10,000 who he said belonged to Nusra Front and al Qaeda.

But he said there should be no rush to military action and called for more time for Russia, Turkey and Iran to discuss the situation in Idlib.

The decision to add Tahrir al-Sham to the list of designated groups was taken by President Tayyip Erdogan. Ankara had already listed Nusra Front as a terrorist group.

Erdogan is due to meet Russian President Vladimir Putin and Iranian President Hassan Rouhani at a three-way summit in Iran on Sept. 7.