BEIRUT, Lebanon — Syria’s ability to produce chemical weapons has been destroyed and its remaining toxic armaments secured, weapons inspectors said Thursday, as President Bashar al-Assad has offered unexpectedly robust cooperation, at least so far, with a Russian-United States accord to dismantle his arsenal.

Elimination of Mr. Assad’s manufacturing ability is the most significant milestone yet in a process that still faces a monumental task: destroying the government’s 1,290 tons of declared chemical weapons in the midst of a bloody civil war that has killed well over 100,000 people and carved up control of the country.

Weapons inspectors who have been in the country just one month say that despite battles raging across the country, deep international disagreement over how to stop the war and even what United States officials say was an Israeli strike on a Syrian Army base late Wednesday night, Syria has so far met all of its commitments and deadlines.

By doing so, Mr. Assad’s government can claim success in what it said would be a key benefit of the accord — seizing a new measure of credibility and portraying itself not as an outlaw regime but as a reliable and legitimate international player. But opponents of Mr. Assad, including the rebels, are deeply critical of the deal for that very reason — it has helped buttress his position but done nothing to stop the war.