President Donald Trump's top officials have declined to comment on the killing of two US citizens, while forcefully defending the president's immigration agenda during a tense congressional hearing.

Facing sharp questions from lawmakers in Congress, the immigration bosses blamed protesters for inflaming rhetoric against agents and said their agencies would not be deterred.

"Let me send a message to anyone who thinks they can intimidate us. You will fail," said Todd Lyons, the acting head of Immigration and Customs Enforcement (ICE).

Tuesday's hearing comes after ICE agents shot and killed Alex Pretti and Renee Good last month, sparking outrage across the country.

Questions for Lyons and Rodney Scott, the commissioner of Customs and Border Protection (CBP), ran the gambit from ongoing activity in Minneapolis to potential future involvement in operations at the voting sites across the US.