An international human rights organization says the Syrian military fired at least four ballistic missiles into the embattled northern city of Aleppo over the past week, killing more than 140 people, including 70 children.

Human Rights Watch says the attacks by the regime of President Bashar Assad on residential areas of Aleppo mark an "escalation of unlawful attacks against Syria's civilian population."

A researcher with the U.S.-based group, who visited Aleppo last week to inspect the targeted sites, said up to 20 buildings were destroyed in each area hit by a missile. There were no signs of any military targets in the residential districts, located in rebel-held parts of Aleppo, the group said in a report Tuesday, Feb 26.

Aleppo has seen some of the heaviest fighting in Syria's nearly 2-year-old conflict.