Moscow on Friday condemned what it said were "completely unacceptable" Israeli strikes on Syria, after a war monitor said the latest air assault had killed more than 40 people.

Russia is Syrian President Bashar al-Assad's key international backer and intervened on his behalf in a bloody civil war.

"Such aggressive actions against the Syrian Arab Republic, which constitute a flagrant violation of the country's sovereignty and the basic norms of international law, are categorically unacceptable," Russia's foreign ministry spokeswoman Maria Zakharova said in a statement.

"We strongly condemn these provocative acts of force, which carry hazardous consequences in terms of a sharp deterioration of the situation in the zone of the Palestinian-Israeli conflict," she added.

The UK-based Syrian Observatory for Human Rights, which has a network of sources inside Syria, said Friday that Israeli air strikes on the country's north had killed 36 soldiers and six Hezbollah fighters.

Contacted by AFP from Jerusalem, the Israeli military said it would "not comment on reports in the foreign media".

Israel has launched hundreds of air strikes in Syria since the civil war there broke out in 2011. It has targeted army positions as well as Iran-backed fighters including Hezbollah, an ally of Damascus, and the Palestinian militant group Hamas.

The strikes have increased since Israel's war with Hamas began on October 7, and Friday's was the second such attack in 24 hours.