(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.
White House press secretary Sarah Sanders did not deny the incident, and instead pushed blame onto Democrats in a statement later Thursday, insisting that her colleagues are "not angry at one another."
"While we are passionate about solving the issue of illegal immigration, we are not angry at one another. However, we are furious at the failure of Congressional Democrats to help us address this growing crisis," she said.
The statement continued, "They should be ashamed for pushing an open borders agenda and are only doing this for strictly political reasons. Despite us having the worst laws in the world and no help from Democrats, our Administration is doing a great job on the border."
This isn't the first time Nielsen's handling of border security has been scrutinized by the Trump White House.
Trump and Nielsen got into a heated argument during a Cabinet meeting in May over border security, a source with knowledge told CNN.
Trump said he didn't think she was doing enough to secure the border and two people told The New York Times, which first reported the argument, that Nielsen drafted a resignation letter over the matter.
While arguing is frequent in this divided West Wing, a source familiar with the clash equated it with a falling out.
Bolton continued on with scheduled meetings throughout the afternoon.
The fight between Kelly and Bolton startled several aides throughout the West Wing on Thursday, a person who heard the shouting said, adding that the raised voices went well beyond the heated discussions of the Trump administration.
After the blowup, aides whispered privately that one of the men might leave the White House given the deep disagreement over the border. The fact that the President sided with Bolton, which only added to Kelly's fury.
A White House official said later that tempers had cooled and Kelly was not going to leave. Another declined to comment.
The caravan issue has been the subject of several White House meetings in recent days, according to an administration official. The conversations have centered around not only how to stop the caravan, but also how to use the issue for the upcoming midterms, the official said. Stephen Miller has been heavily involved.
According to unpublished Department of Homeland Security statistics reviewed by The Washington Post, the number of migrant families entering the US reached record levels in the three months since Trump ended migrant family separations -- a result of the administration's zero tolerance immigration policy.
Earlier this week Trump also took note of a migrant caravan formed in the Honduran city of San Pedro Sula on Saturday and crossed into Guatemala on Monday. The caravan numbers in the thousands, organizers say.
Members of the group told CNN en Español that they decided to join the caravan and head to the United States because of insecurity and a lack of jobs in Honduras. Many are traveling with children in tow.
Trump posted several tweets about the caravan on Tuesday, calling for an end to foreign aid should they continue to travel.
"We have today informed the countries of Honduras, Guatemala and El Salvador that if they allow their citizens, or others, to journey through their borders and up to the United States, with the intention of entering our country illegally, all payments made to them will STOP (END)!" Trump tweeted Tuesday night.
Trump has repeatedly called for cuts to foreign aid in Honduras, Guatemala, El Salvador and Mexico, which, in all, obtained some $260 million in foreign aid for fiscal year 2019.
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