Washington (CNN) A heated argument in the West Wing between chief of staff John Kelly and national security adviser John Bolton over a recent surge in border crossings turned into a shouting match Thursday, two sources familiar with the argument told CNN.

The exchange lay bare a bitter disagreement that has existed between two of President Donald Trump's top aides for weeks now.

Trump, who was incensed about the rising levels of migrants and threatened to shut down the southern border on Twitter earlier that morning, took Bolton's side during the argument. Bolton favors a harder line approach to the issue and criticized Department of Homeland Security Secretary Kirstjen Nielsen during the argument, a source said. Nielsen used to serve as Kelly's deputy when he ran DHS. Bolton reportedly said Nielsen needed to start doing her job, which incensed Kelly.

The President, who sources say was present for the beginning of the shouting match, later denied knowledge of it.

"I've not heard about it. No," Trump told reporters before boarding Air Force One to fly to Montana on Thursday afternoon.

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