WASHINGTON — The United States threatened on Wednesday to halt talks with Russia on the Syria war and scrap plans for joint military targeting of Islamist militants 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.

It came as the Syrians and their Russian allies bombarded insurgent-held areas of eastern Aleppo, the divided northern Syrian city, for the sixth consecutive day in what they have described as a determined effort to eradicate terrorists.

Residents and rescue workers in Aleppo say hundreds of civilians have been killed, many of them children.

Kerry told Lavrov, “The United States is making preparations to suspend US-Russia bilateral engagement on Syria, including on the establishment of the Joint Implementation Center, unless Russia takes immediate steps to end the assault on Aleppo and restore the cessation of hostilities,’’ the State Department said in a statement.

Kerry also “expressed grave concern,’’ to Lavrov, the statement said, about Russian and Syrian attacks on hospitals, the water supply network, and other civilian infrastructure. Some of the airstrikes, Kerry said, included the use of incendiary weapons and bunker-buster bombs, heavy-duty explosives that kill indiscriminately.

The Obama administration has repeatedly said that it is Russia’s responsibility to stop its own attacks and to ensure that Syrian President Bashar Assad complies with the agreement Kerry and Lavrov reached nearly three weeks ago in Geneva to reduce violence in the Syria conflict and allow humanitarian aid into besieged areas.

The two clashed over the conflict in the UN Security Council last week, when Lavrov said the United States had failed to persuade moderate Syrian opposition groups to separate themselves from extremist fighters and abide by a cease-fire.

Kerry said the Russian and Syrian militaries were primarily responsible for the continuing violence, including the Sept. 19 bombing of an aid convoy to Aleppo, which US officials say was carried out by Russian aircraft despite Russian denials.

But until Wednesday, Kerry had said that he was still waiting to hear Russia’s proposals for restoring the cease-fire.

John Kirby, the State Department spokesman, said in a statement, “The secretary stressed that the burden remains on Russia to stop this assault and allow humanitarian access to Aleppo and other areas in need.’’

Even by the standards of the Syrian conflict, in which roughly 500,000 people have been killed since 2011, the violence has soared this year.

Russian and Syrian forces have carried out 2,506 airstrikes since Jan. 1, according to IHS Jane’s Terrorism and Insurgency Center. Forty percent of these strikes, it said, were carried out in the Aleppo region, mainly in Aleppo city.

The Russian and Syrian airstrikes in the Aleppo area have killed at least 1,937 Syrians, more than half of them civilians.