UNITED NATIONS — International efforts to end the war in Syria faltered further on Tuesday as the United Nations mediator quit, citing frustrations over the moribund political negotiations, and France’s top diplomat said there was evidence the Syrian government used chemical weapons more than a dozen times after it signed the treaty banning them.

Taken together, the two events pointed to the failings of the West’s signature efforts on Syria, finding a diplomatic way out of a civil war in its fourth year — and a pact that was proudly touted as stopping the Syrian government from using chemical weapons.

The United Nations secretary-general, Ban Ki-moon, announced that he had accepted the resignation of his special envoy, Lakhdar Brahimi, who told reporters, “It’s very sad that I leave this position and leave Syria behind in such a bad state.”

His departure — without a hint of who might succeed him — signaled the bleak prospects for peace in a conflict that has claimed more than 150,000 lives and shows no signs of abating as President Bashar al-Assad says he intends to serve another seven-year term after staging elections in June. Mr. Brahimi’s announcement came just two days before Secretary of State John Kerry and his counterparts from European and Arab nations are to gather in London to discuss the crisis in Syria, with no new or obvious path forward.