U.S. national security adviser Jake Sullivan warned that China would suffer "real costs" if it decided to provide lethal aid to Russia in its war against Ukraine.

Since invading Ukraine, Russia has regularly sought drones and ammunition from China, according to people with knowledge of the intelligence, and Chinese leadership has been actively discussing whether or not to give lethal assistance for the past several months.

"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," Sullivan told CNN's Dana Bash on "State of the Union." "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's remarks come at a crucial time in the Ukraine conflict. According to reports, the United States has intelligence that the Chinese government is considering assisting Russia with drones and munitions for use in the battle.

In diplomatic discussions with China, he continued, the United States "does not simply issue direct threats." We are simply outlining the stakes and the repercussions, as well as how the situation might play out. And we do so in a transparent and detailed manner behind closed doors."

According to the reports, Beijing has not yet made a final decision, as negotiations between Russia and China regarding the price and scope of the equipment are ongoing.

"I can level with the American people in saying that war is unpredictable," Sullivan said Sunday. "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 Zelensky 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 added.

Sullivan also echoed Biden's remarks on Friday that the administration was "for now" ruling out deploying F-16 fighter jets to Ukraine.

"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, House Foreign Affairs Committee Chairman Michael McCaul stated that Congress "can certainly write into our appropriations bills, prioritizing weapons systems" for Ukraine.