WASHINGTON — President Obama warned President Vladimir V. Putin of Russia on Monday against further disruption of eastern Ukraine even as the United States and Europe prepared to expand sanctions against leading Russian figures in the next few days.

After a weekend of meetings, the White House was working on a list of new targets who would be barred from traveling to the United States or whose assets here would be frozen, officials said. Among them is Igor Sechin, a longtime Putin ally and the president of Rosneft, the largest state-owned Russian oil company, which has a major joint venture with ExxonMobil. European ministers agreed on Monday on a list of their own.

The American sanctions would also be imposed on at least one Russian institution that is deemed part of the so-called crony network supporting Mr. Putin, officials said. But they added that Mr. Obama does not plan to place more crippling measures on whole sectors of the Russian economy unless the Kremlin escalates its actions. The White House wants to hold those sanctions in reserve should Moscow invade Ukraine or seek to annex its eastern regions.

The debate over the Western response came as tensions continued to mount. The Pentagon said Monday that a Russian attack aircraft made 12 close passes near an American destroyer in the Black Sea on Saturday. And the White House confirmed that John O. Brennan, the director of the C.I.A., visited Kiev over the weekend, a move that provided propaganda ammunition to the Kremlin, which has presented the crisis as an American-orchestrated intervention.