BRUSSELS — Promising to surge new aid and weapons to Ukraine, and urging unity as the best strategy to counter Russia’s war against Kyiv, President Joe Biden then went a step further, indicating that trans-Atlantic leaders were prepared to respond “in kind” to a Russian chemical weapons attack.

U.S. officials have warned for weeks that Russian President Vladimir Putin, facing strategic and military setbacks in Ukraine, could soon move beyond regular warfare, tipping the conflict into a dirty war.