Nevertheless, American and British officials have discussed imposing tougher sanctions on people close to Mr. Putin, including some measures that were considered, but put aside, after Moscow’s agents used a nerve agent in an attempt to kill a former Russian intelligence officer in Britain in 2018.

Measures under consideration include blocking Russian oligarchs from using Visa and Mastercard credit cards and restricting where they and their families can travel in Britain and Europe, as well as other kinds of sanctions that might get Mr. Putin’s attention quickly but damage parts of the American or European economy.

A Ukrainian official said the United States was considering a package of increased military aid to Ukraine. (The Biden administration is sending more than $400 million in security assistance to Ukraine this year.) But American officials are hesitant to discuss Washington’s support, for fear of escalating the situation further.