In seven years, the casualties of Syria’s civil war have grown from the first handful of protesters shot by government forces to hundreds of thousands of dead.

But as the war has dragged on, growing more diffuse and complex, many international monitoring groups have essentially stopped counting.

Even the United Nations, which released regular reports on the death toll during the first years of the war, gave its last estimate in 2016 — when it relied on 2014 data, in part — and said that it was virtually impossible to verify how many had died.

At that time, a United Nations official said 400,000 people had been killed.

But so many of the biggest moments of the war have happened since then. In the past two years, the government of President Bashar al-Assad, with Russia’s help, laid siege to residential areas of Aleppo, once the country’s second-largest city, and several other areas controlled by opposition groups, leveling entire neighborhoods. Last weekend, dozens of people died in a suspected chemical attack on a Damascus suburb, prompting the United States, Britain and France to launch retaliatory strikes against Syrian targets early Saturday.