US Chairman of the Joint Chiefs of Staff General Mark Milley assessed military presence on the streets of Washington DC following clashes between protesters and police outside the White House.

Briefly speaking to reporters, the chairman said they support freedom of speech. "We took an oath of allegiance to the constitution of the United States of America to do that - to protect everyone's rights - and that's what we do," he said.

He further said the National Guard forces in Washington, DC are deployed and he is checking how well they are doing.

The US military is sharply increasing the number of National Guard forces in DC to potentially respond to protests over the death of an George Floyd, African-American man, who died in police custody.

Demonstration have erupted across the country over the death of Floyd, a 46-year-old black man who died on May 25 in Minneapolis police custody.

Video footage showed a white Minneapolis police officer kneeling on the neck of Floyd, for nearly nine minutes before he died on May 25. Former Minneapolis police officer, Derek Chauvin, 44, has been charged with third-degree murder.

Demonstration erupted with authorities imposing curfews on dozens of cities across the US.

A tanker truck drove into demonstrators on the I-35 highway in Minneapolis, which had been closed to traffic. Meanwhile, crowds chanting 'George Floyd' also set fires near the White House.

On Monday, the Hennepin County Medical Examiner declared Floyed's death a homicide, saying he died of “cardiopulmonary arrest complicating law enforcement subdual restraint and neck compression.

Protests in US have flared repeatedly in recent years over police killings of black Americans.