AMMAN (Reuters) - Syrian rebel commanders and rights groups said on Monday Russia and its Syrian ally are stepping up aerial strikes on heavily populated cities, in a new and bloodier phase of a three-month-old assault on the last opposition bastion in the northwest.

A sustained week-long aerial attack on cities in southern Idlib province has caused the most civilian casualties since Russian jets joined the Syrian army on April 26 in the offensive to recapture the enclave.

“This is the heaviest bombing and loss of life since the start of the campaign,” said Fadel Abdul Ghany, chairman of the Syrian Network for Human Rights (SNHR).

“They are taking revenge on our popular support that has stood by us,” said Naji Mustafa, spokesman for the Turkey-backed National Liberation Front (NLF) coalition of insurgent groups. He described the raids as “systematic terror by the (President Bashar al-) Assad gangs and Russian forces.”

Both the Syrian government and its Russian ally, whose air power has been critical to military gains by Damascus in recent years, deny targeting civilians or civilian infrastructure.

They say their offensive -- one of the biggest of the eight-year-old conflict -- is intended to end the rule of al Qaeda militants over the northwest.

On Monday jets killed four civilians and wounded scores in a raid on a marketplace in Maarat al-Numan city in the same place where 40 people were killed on July 22 in the single deadliest strike since the campaign began.

The attack followed two days of heavy bombing of the center of Ariha city in which at least 16 civilians were killed, rescuers said.

In all, more than 62 people were killed in four major raids on the cities of Maarat al Numan, Ariha, and several towns since last week, the Syrian Network for Human Rights (SNHR) which documents casualties and briefs various U.N. agencies.

“MOUNTING CIVILIAN CASUALTIES”

U.N. human rights chief Michelle Bachelet on Friday denounced the “apparent international indifference” to the mounting civilian casualties.

Rebels say the campaign has now shifted significantly toward cities and urban centers and away from mainly rural and sparsely populated towns and villages near the frontline.

The heavy bombing of rural villages has so far displaced 440,000 people, who have headed to the safety of areas closer to the border with Turkey, the latest U.N. estimate shows.

The pattern of the latest bombing echoes the relentless strikes that enabled the government to win back former rebel held bastions around the capital last year, rebels said.

“What’s happening now is that ... they are targeting areas that are crowded with people,” said Mohammad Rashid, a rebel official from the mainstream Jaish al Nasr faction.

Rebel commanders expect bombing civilian populations will become a more pressing military priority to crush insurgents, as Russia and its allies become increasingly mired in a campaign that has been costly, with no significant territorial gains.

U.N. officials and aid workers have said a humanitarian disaster could unfold if the strikes expand to Idlib city and other cities that have so far been spared sustained bombing.

A majority of the three to four million people who live in the opposition enclave are in these urban centers and aid workers say wider strikes bring the prospect of a major new refugee exodus along the border with Turkey.