Russian President Vladimir Putin's spokesperson said "sometimes we hear different statements from different agencies" in the U.S.
The White House has not clarified what the stay-behind mission will entail, nor how long it will last.
"She does not have any legal basis, no valid U.S. passport, no right to a passport, nor any visa to travel to the United States," Pompeo said in a statement.
"To those groups who are betting on the Americans, we say the Americans will not protect you," Syrian President Bashar al-Assad said.
About 300 fighters are believed to remain in the devastated Syrian village of Baghouz.
U.S.-backed Kurdish forces are poised to seize control of the remaining ISIS stronghold in Syria.
"I bring greetings from the 45th President of the United States of America, President Donald Trump," Pence told the crowd.
Hundreds of remaining ISIS fighters are engaged in a last stand against U.S.-backed forces in southeastern Syria.
"They want us to kill guys compassionately and only under the rules of engagement that they say to," Representative Duncan Hunter complained.
The lauded song has more than 487 million views on YouTube and 280 million streams on Spotify.
FBI Special Agent in Charge Eric Smith said the suspect represented a threat the agency faces every day.
"I worry that it sends a strong message to our adversaries out there, particularly Russia and China, that there are huge gaps remaining in the national security decision-making apparatus in this country," John Kirby said.
The 21-year-old was charged with five counts of premeditated homicide on Thursday.
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