Biden says he was 'surprised' classified documents were found in his private office

US President Joe Biden said Tuesday that he was surprised to learn that classified documents from his time as vice president were found at the Washington, D.C. office of his think tank.

"People know I take classified documents, classified information, seriously," Biden told a news conference at the North American Leaders' Summit in Mexico City.

He said his lawyers were clearing out his office and found some documents in a box in a locked cabinet and realized there were several classified documents in the box.

Stressing that he does not know what is in the documents, Biden said they were "immediately" handed over to the National Archives.

"They immediately called the Archives, turned them over to the Archives, and I was briefed about this discovery and surprised to learn that there were any government records that were taken there to that office," he added.

Biden's remarks came a day after reports surfaced about classified materials that were discovered in an office at the Penn Biden Center for Diplomacy and Global Engagement in Washington, D.C. on Nov. 2.

The materials included 10 documents covering topics related to Ukraine, Iran and the UK, CNN reported, citing a source.

The documents included US intelligence memos and briefing materials dated between 2013 and 2016, according to the source, and were found in three or four boxes also containing unclassified papers that fall under the Presidential Records Act.

US Attorney General Merrick Garland has received a preliminary report on the documents inquiry and must decide what to do next, including whether to open a full-blown criminal investigation, according to a law enforcement source.

The Federal Bureau of Investigation (FBI) is also involved in the current investigation of Biden's classified materials.