A Syrian opposition spokesman and a war monitor say an airstrike on a training camp in the last rebel enclave in northwestern Syria has killed over 50 fighters.

 Syria's air defences confronted an aerial "aggression" from Israel that targeted an air base in the country's centre late Tuesday, state media reported quoting a military official.

Part of the building in the town of Sarmeen had been turned into a shelter for the displaced, the Britain-based Syrian Observatory for Human Rights said.

The announcement in Baghdad came a day after US Defense Secretary Mark Esper said Washington had carried out military strikes targeting the Iranian-backed Iraqi militia blamed for a rocket attack that killed an American contractor in Iraq last week.

No one claimed responsibility for the attacks, which targeted the Homs oil refinery one of only two in the country as well as two natural gas facilities in different parts of Homs province.

Syrian regime air strikes and artillery fire on Tuesday killed 23 civilians in the last major opposition bastion in the northwest of the country, a war monitor said.

Turkish President Recep Tayyip Erdogan lashed out on Friday at Emmanuel Macron over his criticism of Turkey's military intervention in Syria, saying the French president was suffering "brain death", a phrase Macron recently used to describe NATO.

Pivoting from the dramatic killing of the Islamic State group's leader, the Pentagon is increasing US efforts to protect Syria's oil fields from the extremist group as well as from Syria itself and the country's Russian allies.