LONDON — Nearly one week after a cease-fire was supposed to have gone into effect in Ukraine, Secretary of State John Kerry met with his British counterpart to discuss the imposition of additional sanctions because of what he called Russia’s “brazen” violations of the agreement.

“We know to a certainty what Russia has been providing to the separatists,” Mr. Kerry said at the start of a meeting with Philip Hammond, the British foreign secretary. “We’re not going to sit there and be part of this kind of extraordinarily craven behavior at the expense of the sovereignty and integrity of a nation.”

After their meeting, Mr. Kerry told reporters that “more serious sanctions” were being weighed, and he added that he expected President Obama to make a decision on what steps to take in the next few days. Mr. Kerry did not provide any examples of what sanctions might be chosen or when they might take effect.

Mr. Kerry said the Obama administration had sought to target the sanctions it had already imposed so they would influence the Russian leadership without hurting the Russian public. But in a clear warning to the Kremlin, Mr. Kerry said that sanctions would have a broader effect as they are stepped up, which could lead to more questioning at home of President Vladimir V. Putin’s policies.