US President Joe Biden marked on Monday the "truly grim, heartbreaking milestone" of more than 500,000 deaths in the US due to COVID-19 since the beginning of the pandemic.
Bells tolled from the Washington National Cathedral in tribute to those who have lost their lives.
Biden led a sunset moment of silence and a candle-lighting ceremony at the White House. He ordered American flags lowered at federal buildings for the next five days.
The COVID infection numbers are continuing to increase and are by far the highest reported in the world, accounting for 20 per cent of the nearly 2.5 million coronavirus deaths globally.
Earlier on Monday, he paid tribute to victims of the deadly pandemic and their loved ones.
"The people we lost were extraordinary. They spanned generations," Biden said. "Born in America, emigrated to America. But just like that, so many of them took their final breath alone in America," he said.
"While we've been fighting this pandemic for so long, we have to resist becoming numb to the sorrow," Biden said. "We have to resist viewing each life as this, as a statistic, or a blur, or on the news. And we must do so to honour the dead, but equally important, care for the living those who are left behind."
The half-million milestone comes as states redouble efforts to distribute the coronavirus vaccine after last week's winter weather closed clinics, slowed vaccine deliveries, and forced tens of thousands of people to miss their shots.