German Chancellor Angela Merkel with President Donald Trump during a joint news conference in the East Room of the White House. Thomson Reuters A senior German lawmaker has expressed concern about reports that President Donald Trump revealed highly classified information about the Islamic State terrorist group to Russian officials.

Burkhard Lischka said in a statement to the Associated Press that "if it proves to be true that the American president passed on internal intelligence matters, that would be highly worrying."

Lischka, who sits on the German parliament's intelligence oversight committee, noted that Trump had access to "exclusive and highly sensitive information including in the area of combating terrorism."

The Social Democratic Party lawmaker said that if the US president "passes this information to other governments at will, then Trump becomes a security risk for the entire Western world."

Germany is heavily dependent on US intelligence.

Trump used Twitter on Tuesday morning to defend his sharing of information with the Russians. He said he wanted to share with Russia "facts pertaining to terrorism and airline flight safety." He noted that as president, he has an "absolute right" to do this.

The Washington Post reported Monday that Trump divulged highly classified "code-word" information that could enable the Russians to trace the source of the intelligence.

Trump added a line in his tweet suggesting why he did it: "Humanitarian reasons, plus I want Russia to greatly step up their fight against ISIS & terrorism."

The disclosure late Monday drew strong condemnation from Democrats and a rare rebuke of Trump from some Republican lawmakers. White House officials denounced the report, saying the president did not disclose intelligence sources or methods to the Russians, though officials did not deny that classified information was disclosed in the Wednesday meeting.

The Kremlin has dismissed the reports as "complete nonsense."

Russian presidential spokesman Dmitry Peskov on Tuesday dismissed the reports as "yet more nonsense" and said Moscow didn't "want to have to do anything with it," adding that "there is nothing to confirm or deny." Maria Zakharova, the spokeswoman for the Russian foreign ministry, on Facebook on Tuesday described the reports as "yet another fake."

American national security experts reacted with alarm to the news. Trump, as president, is legally allowed to essentially disclose classified information to whomever he wants. But the fact that the information he shared was not a US secret, but that of an American ally, may complicate his authority to declassify information at will.