James Mattis
James Mattis Reuters

U.S. Defense Secretary James Mattis said on Tuesday he is "not worried" that President Donald Trump disclosed classified information during discussions with senior Russian leaders.

Speaking briefly with reporters outside the Pentagon after the new Air Force secretary was sworn in and quoted by The Associated Press, Mattis said he had discussions Tuesday with three allies, including two who are NATO members, and the issue "never even came up."

He said he did not have any details about Trump's conversation.

The Washington Post, citing unnamed sources, on Monday reported that Trump revealed highly classified information to the Russian foreign minister and ambassador in a White House meeting last week.

The sources claimed Trump’s disclosures jeopardized a critical source of intelligence on the Islamic State (ISIS) jihadist group.

Trump responded to the reports via his Twitter account on Tuesday, saying, "As President I wanted to share with Russia (at an openly scheduled W.H. meeting) which I have the absolute right to do, facts pertaining to terrorism and airline flight safety."

He noted that the information he shared with the Russian foreign minister is connected to the fight against terrorism and to air security.

Later on Tuesday, The New York Times claimed that Israel was the source of the intelligence that Trump shared with the Russian officials.

Israeli officials refused to comment on the report that it was Israeli intelligence which was shared with Russia. Israel's Ambassador to the U.S., Ron Dermer, told the newspaper that the two nations would continue to cooperate closely in the field of counter-terrorism.