Syria's conflict has now claimed nearly 93,000 lives, the UN human rights commission has said, but it warned that the true death toll after 27 months of violence was likely to be higher.

Amid warnings of a new government offensive against rebels in parts of Aleppo, the UN said the figure of 92,901 was reached at the end of April, with an average of over 5,000 people being killed every month since July last year. More killings, however, may be undocumented.

The UN statistics also showed that at least 6,561 minors were among the dead, prompting calls for urgent humanitarian access to prevent further "appalling abuses". Of those, 1,729 were reported to be under the age of 10. The figures were "heartbreaking", said Unicef UK, warning of a "lost generation" in Syria.

"This extremely high rate of killings, month after month, reflects the drastically deteriorating pattern of the conflict over the past year," Navi Pillay, the UN's high commissioner for human rights, said in a statement.

The latest statistics are published against a background of international efforts to convene peace talks between Bashar al-Assad's government and rebels seeking to overthrow him – as well as intensifying discussion in the west about supplying weapons to opposition fighters if negotiations fail. Syria is high on the agenda of the G8 summit in Northern Ireland, which will bring Barack Obama and Vladimir Putin together next week.

The previous UN figure, issued in mid-May, was that 80,000 people had been killed in the conflict, which began with peaceful and initially localised protests against the Assad regime in March 2011 and turned into a countrywide armed rebellion a few months later.

The latest analysis is based on data from eight sources, including the Syrian government and the British-based Syrian Observatory for Human Rights (Syria HR). Killings were included only if the name of the victim and the date and location of death were known. "The status of the victims as combatants or noncombatants is unknown for all but a few records," says the report by the Human Rights Data Analysis Group.

"This report finds that when the fully identified records were combined and duplicates identified, the eight databases collected here identified 92,901 unique killings. The enumeration is likely undercounting the true total number of conflict-related killings that have occurred during this time period. This is because an unknown number of conflict-related killings are likely to have occurred without being documented anywhere."

Pillay added: "There are also well-documented cases of individual children being tortured and executed, and entire families, including babies, being massacred – which, along with this devastatingly high death toll, is a terrible reminder of just how vicious this conflict has become."

Save the Children said in a statement: "The appalling abuses of children documented by the United Nations underline the urgent need for a resolution to this conflict and for humanitarian access to those children still trapped in the country. It is a sad indictment of the international community's failure on Syria that children continue to be killed, injured and maimed by artillery and crossfire.

"As the G8 prepares to meet in Northern Ireland, this is a timely reminder of the devastating effect of this conflict on the young. We support the UN view that 1,700 under-ten deaths is likely to be a huge underestimate, as two thirds of all recorded deaths do not include an age."

Pillay also warned of an impending Syrian government offensive against Aleppo following the defeat of rebel forces in Qusair, near Homs, last week.

Activists on Thursday reported rebels gaining control of a key military base in the central Hama province straddling the country's strategic north-south highway leading to the province of Aleppo.

Syria HR said the rebels seized the base after clashes with regime forces. It said the rebels killed six government troops and seized weapons and ammunition. Assad's forces are waging an offensive to drive rebels out of Hama, Homs and Aleppo.