A "track and trace" system is set to start in England and Scotland on Thursday.
UK Health Secretary Matt Hancock says he believes the "vast majority of people" will self-isolate if told to.
The US has recorded more than 100,000 deaths from Covid-19, figures from Johns Hopkins University show.
Its 1.69m confirmed infections account for about 30% of the worldwide total.
New York City, especially hard hit, is seeing a drop in the death rate but one study shows it rising in 20 states.
A row over the UK PM's aide's personal travel during lockdown is continuing.
In South Korea, a new cluster has been growing linked to a logistics centre outside Seoul.
There have been more than 5,690,000 confirmed virus cases worldwide and 355,000 deaths, according to JHU.
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