BRUSSELS—Russia’s role in Libya is causing growing concern at the North Atlantic Treaty Organization, a senior official said Saturday, with the Kremlin appearing to throw its support behind Gen. Khalifa Haftar, a rival of the United Nations-backed coalition government in Tripoli.

“I am very concerned about Russian forces seemingly gathering to influence the situation there. It troubles me very much,” said Rose Gottemoeller, a deputy secretary general at NATO.

Speaking at the Brussels Forum annual foreign policy conference, she noted that Russia had backed the United Nations Security Council resolution aimed at gathering international support behind the fragile national unity government. However, she said it seemed that “there was a decision made in the Kremlin to simply toss out that” resolution.

“The fact that they have turned to the General now—to General Haftar—and they’re putting an emphasis on working with him…that’s not the attempt at establishing a government of national unity that was established by the U.N. Security Council resolution,” she told reporters later.

European countries have lobbied Russia to use its influence with Gen. Haftar, who has received support from Egypt, the United Arab Emirates and Saudi Arabia, to get him to reconcile with the U.N.-backed Tripoli Government of National Accord, led by Faiez Serraj.