Russian warplanes have killed at least 18 people in a town in northern Syria held by insurgents, a monitoring group reported, as Turkey prepares to hand over the body of a Russian pilot shot down in Turkish air space.

The airstrikes were on the town of Ariha, according to the Syrian Observatory for Human Rights (SOHR), and the Local Coordination Committees, an activist collective, said a busy market was hit, causing heavy casualties.

The SOHR, which relies on a network of activists inside Syria, said the airstrike destroyed three buildings in the town centre. Ariha Today, a Facebook page that covers the town, said the airstrikes were carried out by Russian planes and claimed that 40 people were killed and more than 70 wounded.

If it is confirmed that Russian aircraft carried out the attack, it would be one of the deadliest incidents since Moscow began launching airstrikes in Syria two months ago. The SOHR says Russian strikes have killed more than 400 civilians, including 166 women and children.

News of the bombing came as the body of the pilot killed when Turkey shot down a Russian jet was taken to Turkey late on Saturday to be handed over to Russia, the Turkish prime minister said.

The body was being treated in accordance with the Orthodox tradition, Ahmet Davutoğlu told a news conference in Ankara on Sunday before going to Brussels for a meeting with EU leaders on migration.

He did not say how the body of the dead pilot, who Russia has identified as Lt Col Oleg Peshkov, was delivered to Hatay in southern Turkey. But he said Russia’s military attache was going there on Sunday as part of procedures to recover the remains.

Relations between Turkey and Russia have sharply deteriorated since the incident on Tuesday, with Moscow imposing economic sanctions and revoking a visa agreement, while Turkey has sought to cool tensions, seeing the Paris climate change talks that start this week as a chance to mend ties.

Davutoğlu said that with different coalitions operating in Syria with differing objectives, similar incidents to that of the downing of the Russian jet could happen unless there was information sharing and coordination.

The US along with other allies, including Turkey, are also carrying out air campaigns against Islamic State and other groups. Russia and Turkey have accused each other of aiding Isis, but both say they are battling the militants who have taken over swaths of land in Iraq and Syria.

However, Russia sides with the Syrian president, Bashar al-Assad, and has been accused by the west of using its bombing campaign to help him instead of targeting Isis. Turkey wants Assad gone and has been supporting rebels fighting him.

Assad told a senior Iranian official on Sunday that his adversaries had increased weapon supplies and financial support to insurgents since the start of a major offensive aided by his allies to regain lost territory.

He was quoted by state media as telling Ali Akbar Velayati, a top adviser to Iran’s supreme leader, that the military support his country was getting from Iran and Russia had pushed other states, which he did not name, to “further escalate and increase financing and equipping of terrorists”.

Britain is likely to hold a parliamentary vote on extending the bombing of Isis to Syria on Wednesday.