US Homeland Security Secretary Chad Wolf has said federal agents "will not retreat" in their efforts to protect government buildings in Portland.
"If you are a violent rioter looking to inflict damage to federal property or law enforcement officers, you need to find another line of work," he said.
US President Donald Trump sent federal agents to Portland in Oregon to end weeks of anti-racism protests there.
But the mayor of Portland has called for them to leave the US city.
There have been nightly protests against police brutality in Portland since the death of George Floyd, an unarmed black man, in Minnesota in May.
In recent days, however, violent clashes between demonstrators and federal law enforcement officers have escalated.
What did Chad Wolf say about the violence?
Speaking during a press conference on Tuesday, Mr Wolf said federal law enforcement officers in Portland were only targeting and arresting demonstrators who had been identified as being involved in "criminal activity".
He said the department respected the right of people to protest peacefully, but urged demonstrators to "please do so away from the violent activity taking place near the courthouse on a nightly basis".
On Monday evening, federal officers fired tear gas to disperse large crowds of protesters – some armed with hammers – who had gathered outside the city's courthouse and were throwing projectiles.
Mr Wolf also denied claims that the security officers had no identification and insisted they were wearing insignia showing they were police.
"These officers are not military, they are civilian police officers," he said, adding that they were required to restore order following "violent criminal activity every single night for 52 nights" and in response to "a lack of action from city officials".
"We will continue to take the appropriate action to protect our facilities and our law enforcement officers," he said, adding that federal agents will leave Portland when the violence stops.
What's been happening in Portland?
The last week has seen a violent escalation between protesters and federal agents, deployed two weeks ago by Mr Trump to quell civil unrest.
A report from Oregon Public Broadcasting (OPB) this week contained detailed accounts of witnesses who had seen federal law enforcement officers dressed in camouflage emerge from unmarked vehicles, grab protesters without explanation, and drive off.
Mr Wolf said on Tuesday that federal officers had arrested 43 people in Portland since 4 July.
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At the weekend, fencing erected around the federal courthouse to protect the building was dismantled by protesters just hours after it was put up.
The prosecutor's office in Oregon Read More – Source
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