WASHINGTON — The United States threatened on Wednesday to halt talks with Russia on the war in Syria and scrap plans for joint military targeting of jihadists unless the Russian and Syrian militaries stopped bombing Aleppo.

The threat, conveyed via telephone by Secretary of State John Kerry to his Russian counterpart, Foreign Minister Sergey V. Lavrov, was the sharpest warning the Americans have made to the Russians over Syria since their Sept. 9 collaboration on a cease-fire collapsed last week.

But the Obama administration did not say what steps it would take if diplomatic efforts failed entirely or how it hoped to deter Russian and Syrian forces from their offensive against rebels in Aleppo, the divided northern Syria city that has become a focal point of the war.

Mr. Kerry’s warning came as the Syrians and their Russian allies bombarded insurgent-held areas of eastern Aleppo for the sixth consecutive day in what they have described as a determined effort to eradicate terrorists.