Since late April, the Syrian regime and Russia have stepped up deadly raids on the Idlib region of three million people.

The bombardment on the jihadist-held bastion has killed hundreds, pounded health centres and schools, and caused more than 330,000 people to flee their homes.

Aid groups have decried a "nightmare" that has slain an alarming amount of children, in the latest bloody episode of Syria's eight-year civil war.

Save the Children said the number of children killed in Idlib over the past four weeks had exceeded the number slain in the same region in the whole of last year.

"The current situation in Idlib is a nightmare," the charity's Sonia Khush said.

"It's clear that once again children have been killed and injured in indiscriminate attacks," she added in a statement.

On Thursday, Russian air strikes killed seven civilians -- including one child -- in the bastion, most in and around the town of Maaret al-Numan in Idlib province, the Syrian Observatory for Human Rights said.

And regime air raids killed five civilians -- including two children -- in neighbouring Aleppo province, the Britain-based monitor said.

The bastion under aerial attack is made up of a large part of the Idlib province, as well as slivers of the adjacent governorates of Aleppo, Hama, and Latakia.

More than 730 civilians have been killed there in air strikes and ground-to-ground fire by the Damascus government and its allies since late April.

The bombardment has increased in intensity in recent weeks, with the toll of almost 50 civilians on Monday alone -- the majority in air strikes on a busy market.

Syria's political opposition Thursday condemned "on average at least a massacre a day" in the embattled stronghold.

"The Syrian people are really being targeted by a genocide," the head of the Syrian National Coalition Anas Abdah said at a press conference in Istanbul.

A September accord struck between Moscow and Ankara was supposed to protect the region, but it was never fully implemented after jihadists refused to withdraw from a planned buffer zone.

In January, Hayat Tahrir al-Sham -- a group led by Syria's former Al--Qaeda affiliate -- took over administrative control.

Outside the bastion, rebel and jihadist rocket fire has also killed non-combattants -- but in far lower numbers.

Since late April, retaliatory rocket fire has killed around 70 civilians in nearby government-held territory.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-government protests.