Bolton spoke to White House reporters while holding the yellow notepad and discussing the crisis in Venezuela, where the US now recognizes opposition leader Juan Guaido as the country's interim president.

It was not until after the briefing that observers spotted the black scrawl.

Speaking on condition of anonymity, a US official said "we are not seeing anything that would support" a potential troop deployment to Colombia, which neighbors Venezuela.

The Pentagon referred a query back to the White House.

During the briefing, Bolton would not rule out use of US troops in Venezuela.

"The president has made it clear on this matter that all options are on the table," he said.

The US military's Southern Command did not immediately respond to requests for comment.

Bolton's notepad also had the line: "Afghanistan -- welcome the talks" -- a reference to a potential breakthrough in discussions with the Taliban.