BRUSSELS — The United States and Europe agreed on Thursday to place additional economic sanctions on Russia over its role in promoting separatist warfare in eastern Ukraine despite a fragile new cease-fire that went into effect last week.

European leaders overcame resistance among some of their colleagues who wanted to continue delaying additional measures to see if the shaky truce, negotiated in Minsk, Belarus, holds. President Obama then followed suit shortly afterward by announcing that he would match the new European moves when they take effect on Friday.

“We are watching closely developments since the announcement of the cease-fire and agreement in Minsk, but we have yet to see conclusive evidence that Russia has ceased its efforts to destabilize Ukraine,” Mr. Obama said. He added: “If Russia fully implements its commitments, these sanctions can be rolled back. If instead Russia continues its aggressive actions and violations of international law, the costs will continue to rise.”