U.S.-based rights group Human Rights Watch on Thursday accused Syria of war crimes by indiscriminate and sometimes deliberate airstrikes against civilians, killing at least 4,300 people since last summer.

In recent months, parts of northern Syria, especially areas along the border with Turkey, have fallen under the control of rebels, including several neighbourhoods of Aleppo, the country’s largest city.

“The aim of the airstrikes appears to be to terrorise civilians from the air, particularly in the opposition-controlled areas where they would otherwise be fairly safe from any effects of fighting,” Ole Solvang of the New York-based group told The Associated Press.

These attacks are “serious violations of international humanitarian law”, and people who commit such breaches are “responsible for war crimes”, Human Rights Watch said.

Mr. Solvang led the HRW team that inspected 52 sites in northern Syria and documented what it said were 59 unlawful attacks by the Syrian Air Force. At least 152 people were killed in these attacks, according to a HRW report released on Thursday.

The group inspected sites only in rebel-held areas because the Syrian government barred access to parts of the country under its control.

Based on inspections and more than 140 interviews with witnesses, HRW said warplanes “deliberately targeted four bakeries (in the north) where civilians were waiting in bread lines a total of eight times.”

Repeated aerial attacks on two hospitals in the areas the group visited in the northern areas under opposition control “strongly suggest that the government also deliberately targeted these facilities,” HRW said.

At the time of Human Rights Watch visits to the two hospitals, the facilities were attacked seven times, the group said.

In most of the strikes, the Syrian planes appeared to have had no military target in sight, such as armed opposition supporters or rebel headquarters, when they dropped their explosives on civilian areas, the group said.

In addition to the attacks on the bakeries and hospitals, HRW documented 44 other cases in which it concluded that airstrikes were “unlawful under the laws of war,” saying that the regime forces dropped “imprecise and inherently indiscriminate” munitions, including cluster bombs, from high-flying helicopters and jets on civilian areas.

The 80-page HRW report said that across Syria, more than 4,300 civilians have been killed in air attacks since last July.

The report is the most comprehensive study of Syrian air force operations since last summer, when the forces of President Bashar Assad started to rely heavily on fighter jets to repel rebel advances and reverse their territorial gains.

Officials in Damascus could not immediately be reached for comment on the report.

The opposition now controls large swaths of northern Syria and last month captured their first provincial capital, Raqqa. The opposition fighters also control districts of Aleppo and some key infrastructure in the east, including oil fields and dams on the Euphrates River.

With the recent influx of better weapons and other foreign aid, the rebels have also made major gains in the south, seizing military bases and towns in the strategically important region between Damascus and the border with Jordan, about 160 km from the capital.

While the rebels have made major gains, often they cannot hold on to the territory because of the regime’s superior air power. The continued threat from the air has also stalled efforts to effectively govern rebel-held areas, allowing opposition leaders from the Western-backed alliance only brief forays into areas under rebel control.

“The air force has no other reason to continue hitting rebel areas but to prove to the opposition that it can’t rule,” Mr. Solvang said in a phone interview on Thursday.

For its part, the opposition Syrian National Coalition has been marred by severe divisions in its ranks since its formation late last year in Qatar, and its leaders are mostly seen as disconnected from the myriad rebel forces fighting inside Syria.

U.S. Secretary of State John Kerry met with opposition leaders in London on Wednesday to discuss ways to step up aid to rebel forces.

The United Nations says that more than 70,000 people have been killed in Syria’s two-year civil war.

More In: World | International | News