BEIRUT: Clashes, shelling and air raids in western Syria marred a Russian- and Turkish-backed cease-fire that aims to end nearly six years of war and lead to peace talks between rebels and a government emboldened by recent battlefield success.

Russian President Vladimir Putin, a key ally of Syrian President Bashar Assad, announced the cease-fire Thursday after forging the agreement with Turkey, a longtime backer of the opposition.

The truce went into force at midnight but monitors and rebels reported almost immediate clashes, and violence appeared to escalate later Friday as warplanes bombed areas in the country’s northwest, they said.

Asaad Hanna, a political officer in the Free Syrian Army, a loose alliance of insurgent groups, told Reuters violence had reduced but had not stopped.

“We cannot be optimistic about someone like the Russians who used to kill us for six years ... they are not angels. But we are happy because we are reducing the violence and working to find a solution for the current situation,” Hanna said.

The cease-fire is meant as a first step toward fresh peace talks, after several failed international efforts this year to halt the conflict. The agreement brokered by Russia and Turkey, which said they would guarantee the truce, is the first of three such deals this year not to involve the United States or United Nations.

Moscow is keen to push ahead with peace talks, hosted by its ally Kazakhstan. But the first challenge will be maintaining the truce, which looked shaky Friday.

Syrian government warplanes carried out nearly 20 raids against rebels in several towns along the provincial boundary between Idlib and Hama, the British-based Syrian Observatory for Human Rights said. Clashes between rebel groups and government forces took place overnight in the area, the Observatory and rebel officials said. Warplanes and helicopters also struck northwest of Damascus in the rebel-held Wadi Barada valley, where government troops and allied forces clashed with rebels, the Observatory reported.

A military media unit run by Damascus’ ally Hezbollah denied any Syrian government airstrikes on the area.

An official from the Nour al-Din al-Zinki rebel group said government forces had also tried to advance in southern Aleppo province.

Residents of several rebel-held areas, including towns and cities in Idlib province, used the relative calm to hold street protests against the Syrian government Friday, the Observatory said.

A number of rebel groups have signed the new agreement, Russia’s Defense Ministry said Thursday. Several rebel officials acknowledged the deal, and an FSA spokesman said it would abide by the truce.

Hanna, the FSA political officer, said late Friday rebels were not yet responding to attacks by pro-government forces and had asked Turkey to make sure the attacks stop.

“If the breaks come again, we will reply to all the sources of fire. We are monitoring the fighting but our weapons are ready,” he said.

Friday the Russian ambassador to the United Nations said Moscow had circulated a proposed resolution at the U.N. Security Council that would endorse the cease-fire, and said he hoped the council would vote on the resolution Saturday.

In another sign that the latest truce could be as challenging to maintain as its predecessors, there was confusion over which rebel groups would be covered by the cease-fire.

The Syrian army said the agreement did not include the radical Islamist group Daesh (ISIS), fighters from Al-Qaeda’s former branch the Nusra Front, or any factions linked to those extremist groups.

But several rebel officials said Thursday that the agreement did include the former Nusra Front – now known as Jabhat Fatah al-Sham – which announced in July that it was severing ties with Al-Qaeda.

A spokesman for Jabhat Fatah al-Sham criticized the cease-fire for not mentioning Assad’s fate, and said the political solution under this agreement would “reproduce the criminal regime.” “The solution is to topple the criminal regime militarily,” he said in a statement Friday.

The powerful Islamist insurgent group Ahrar al-Sham said it had not signed the cease-fire agreement because of “reservations” but did not elaborate.

The deal follows a thaw in ties between Russia and Turkey.

Ankara backs rebels fighting against Daesh, which has made enemies of all other sides involved in the conflict.

In a sign of the detente, the Turkish armed forces said Friday Russian aircraft had carried out three airstrikes against Daesh in the area of Al-Bab in northern Syria.

Ankara has insisted on the departure of Assad but his removal has become a secondary concern to fighting the expansion of Kurdish influence in northern Syria. The chances of Assad’s opponents forcing him from power now seem more remote than at any point in the war.

The United States, in the waning days of U.S. President Barack Obama’s administration, has been sidelined in recent negotiations and is not due take part in the Kazakhstan talks.

Russia has said the United States could join a fresh peace process once President-elect Donald Trump takes office on Jan. 20. It also wants Egypt to join, together with Saudi Arabia, Qatar, Iraq, Jordan and the United Nations.