LONDON — An opposition monitoring group that has tracked Syria’s widening civil war said Wednesday that more than 100,000 people had died in the 27-month-old conflict, with pro-government forces taking far more casualties than rebels seeking the overthrow of President Bashar al-Assad, while civilians accounted for more than one-third of the overall fatalities, the biggest single category.

The group, the Syrian Observatory for Human Rights, which is based in Britain and relies on a network of activists in Syria for its information, put the total number of dead at 100,191 since the Syrian revolt began in March 2011. That is several thousand more than the newest United Nations count of almost 93,000 by the end of April, a number distilled from a pool of 263,055 reported killings by researchers who eliminated those lacking detail and crosschecked to remove duplicate reports. The final number, the researchers and the United Nations said, was therefore conservative.

Rami Abdul Rahman, the founder of the Syrian Observatory, said in a telephone interview that his number — 100,191 — came from adding together the daily tallies his organization has kept since the beginning of the uprising.

Those tallies were based on information from sources including the activists on the ground in Syria, lawyers, and health workers in civilian and military hospitals, rather than from combatants whose estimates could be unreliable.