(CNN) — US national security adviser Jake Sullivan on Sunday vowed there would be “real costs” for China if the country went forward with providing lethal aid to Russia in its war on Ukraine.

“From our perspective, actually, this war presents real complications for Beijing. And Beijing will have to make its own decisions about how it proceeds, whether it provides military assistance. But, if it goes down that road, it will come at real costs to China. And I think China’s leaders are weighing that as they make their decisions,” Sullivan told CNN’s Dana Bash on “State of the Union.”

In diplomatic conversations with China, he added, the US is “not just making direct threats. We’re just laying out both the stakes and the consequences, how things would unfold. And we are doing that clearly and specifically behind closed doors.”

Sullivan’s comments come at a critical juncture in the war in Ukraine. The US has intelligence that the Chinese government is considering providing Russia with drones and ammunition for use in the war, three sources familiar with the intelligence told CNN.

It does not appear that Beijing has made a final decision yet, the sources said, as negotiations between Russia and China about the price and scope of the equipment are ongoing.

Since invading Ukraine, Russia has repeatedly requested drones and ammunition from China, the sources familiar with the intelligence said, and Chinese leadership has been actively debating over the last several months whether or not to send the lethal aid, the sources added.

“I can level with the American people in saying that war is unpredictable,” Sullivan said Sunday when asked if the US could continue supporting Ukraine at current levels a year from now. “One year ago, we were all bracing for the fall of Kyiv in a matter — in a matter of days. One year later, Joe Biden was standing with President Zelenskyy in Kyiv declaring that Kyiv stands.”

“So, I cannot predict the future, and nor can anyone else. And anyone who is suggesting they can define for you how and when this war will end is not leveling with the American people or anyone else,” he said.

Sullivan also reiterated Biden’s Friday remarks that the administration was ruling out providing F-16 fighter jets to Ukraine “for now.”

“This phase of the war requires tanks, infantry fighting vehicles, armored personnel carriers, artillery, tactical air defense systems, so that Ukrainian fighters can retake territory that Russia currently occupies,” Sullivan said. “F-16s are a question for a later time.”

Sunday also marked the nine-year anniversary of Russia’s occupation of the Ukrainian territory of Crimea. The US State Department on Sunday reasserted that “Crimea is Ukraine.”

“The United States does not and never will recognize Russia’s purported annexation of the peninsula,” department spokesperson Ned Price said in a statement, calling Russia’s 2014 seizure of Crimea “a clear violation of international law and of Ukraine’s sovereignty and territorial integrity.”

Sullivan, however, would not say whether the Biden administration would support Ukraine deciding that victory would mean retaking Crimea.

“What ultimately happens with Crimea, in the context of this war and a settlement of this war, is something for the Ukrainians to determine with the support of the United States,” he said to Bash.

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