US President Joe Biden expressed worries about North Korea's "illicit" nuclear and missile programs during his summit with Chinese President Xi Jinping in California this week, a senior US official said Thursday.

National Security Council Senior Director for China and Taiwan Affairs Sarah Beran made the remarks during an online press briefing on the summit that took place at the Filoli estate in Woodside, south of San Francisco, on Wednesday.

"He raised concerns regarding the DPRK's illicit ballistic and nuclear program," she said, referring to the North by its official name, the Democratic People's Republic of Korea.

Beran said that in a number of different engagements with Chinese officials, the United States has raised a range of North Korean issues, including burgeoning military cooperation between Pyongyang and Moscow.

"We've focused on our concerns with the growing relationship between the DPRK and Russia, what that means for denuclearization of the peninsula, what that could mean for Russia's U.N. Security Council obligations and the potential for another nuclear test," she said.

"I think we've been pretty consistent in raising those concerns and urging the Chinese to think seriously about what it means, what the impact would be on regional security as well," she added.

During the briefing, Assistant Secretary of State for East Asian and Pacific Affairs Daniel Kritenbrink also said US officials have continuously talked about North Korea-related concerns with Chinese officials and noted Beijing's role in deterring North Korea's threats and encouraging it to return to dialogue.

"It is not surprising that we would continue to raise these issues with our Chinese counterparts," he said.

Wednesday's summit between Biden and Xi dealt with a wide range of issues, including the war in Ukraine, the war between Israel and the Hamas militant group, human rights and the South China Sea, as well as Taiwan, a self-ruling democracy that China regards as its territory.

The two sides agreed to resume bilateral military communications and curb illicit fentanyl production in a display of their desire to stabilize the taut relationship between the two superpowers.

The meeting between Biden and Xi was their second in-person talks following their last summit during the Group of 20 summit in Bali, Indonesia, last November. It also marks their seventh interaction since Biden took office in January 2021. (Yonhap)