AMMAN (Reuters) – Russian warplanes bombed the rebel-held northwest, on Sunday, in the deadliest strikes since a Turkish-Russian deal halted major fighting with a ceasefire nearly six months ago, opposition sources said.

Eyewitnesses said that warplanes bombed the western outskirts of the city of Idlib, and that heavy artillery shelled the Jabal Al-Zawiya area in southern Idlib from locations close to the Syrian army. There were no immediate reports of injuries.

“These 30 strikes are by far the heaviest strikes so far since the ceasefire agreement,” said Mohamed Rashid, a former opposition official and volunteer plane observer whose network covers the Russian air base in the western coastal province of Latakia.

Other tracking centers said that Russian Sukhoi warplanes bombed the Horsh area and the town of Arab Saeed, west of Idlib. In addition, unidentified drones struck two towns controlled by the rebels in the Al-Ghab Plain, west of Hama Governorate.

There has been no large-scale aerial bombardment since the March agreement ended a Russian-backed bombing campaign that has displaced more than a million people in the region bordering Turkey after months of fighting.

There was no immediate comment from Moscow or the Syrian army, which has long accused armed groups with influence in the last opposition stronghold of undermining the ceasefire and attacking areas controlled by the army.

The agreement between Turkish President Recep Tayyip Erdogan and Russian President Vladimir Putin also led to defuse the military confrontation between them after Ankara paid thousands of troops to Idlib province to repel the Russian-backed forces from any new progress.

Western diplomats who are following the situation in Syria say Moscow stepped up pressure on Ankara in the last round of talks on Wednesday to reduce its heavy military presence in Idlib. Turkey has more than ten thousand soldiers stationed in dozens of bases there, according to opposition sources in contact with the Turkish army.

Eyewitnesses say that the intermittent shelling of the Syrian army’s positions against the Turkish bases witnessed an escalation in the past two weeks. The rebels say the Syrian army and allied militias are massing forces on the front lines.

Two witnesses said that a Turkish military convoy containing at least 15 armored vehicles was seen at night enter Syria through the Kafrlossin border crossing towards a main base in Idlib countryside.