President Donald Trump’s frustration over rising numbers of undocumented immigrants crossing into the U.S. is stoking tensions among his advisers as some White House officials push for a more aggressive policy that includes completely sealing the southern border, said four people familiar with the discussions.

The differences escalated into an angry, profanity-laced exchange on Thursday between White House Chief of Staff John Kelly and national security adviser John Bolton as a Honduran migrant caravan of roughly 4,000 people approached the U.S. border. The dispute was so heated, according to several people, that Kelly ended up storming out of the White House shortly afterward.

Trump’s hard-line stance on immigration, including family separation policies that involved holding children in cages, has been a centerpiece of his presidency and a core message ahead of the midterm elections. But his failure to secure funding for a promised wall along the border with Mexico, coupled with a renewed surge in border crossings, has tested his ability to make good on his promises.