Larry Kudlow, President Donald Trump's top economic advisor, told reporters Friday that he only found out about the latest proposed tariffs against China on Thursday night.

Trump proposed tariffs Thursday on an additional $100 billion in imports from China. The president has previously called for tariffs on steel and aluminum imports, as well as duties on $50 billion in other Chinese goods.

Stock markets declined Friday. The Dow Jones industrial average was down more than 700 points, or about 3 percent, Friday afternoon.

When the Dow was down about 100 points, Kudlow told reporters he thought markets might take the latest trade developments harder than they had. The former CNBC senior contributor has repeatedly said the tariffs may not even go into effect and that they're an early part of a negotiation process between the two nations.

Later in the day, he insisted the president was not using tariffs as just a "negotiating card." Kudlow said the U.S. may give China a list of suggestions for trade policy it wants to see change.

Asked about Treasury Secretary Steven Mnuchin's comment Friday that a trade war is possible, Kudlow said "we are not in a trade war." White House press secretary Sarah Huckabee Sanders, meanwhile, said Thursday that Trump "absolutely" still believes trade wars are easy to win.

Kudlow has increasingly become the face of the administration's response to investors' concerns over Trump's trade policies. A former Wall Street economist and aide in the Reagan administration, Kudlow is known for his free-trade views, but he has pushed the president's more protectionist policies, arguing that China needs to be more open and fair.

He told Bloomberg News on Friday that tariff negotiations had not actually begun yet, and that he believes China's response to the administration's complaints "has been unsatisfactory."

China swiftly pushed back against Trump's new tariffs.

"We will immediately fight back with a major response," a representative of China's Ministry of Commerce said Friday morning. "We have no other choice."

CNBC's Jacob Pramuk contributed to this report.