US tech giants Apple and Google said on Wednesday they were offering health authorities around the world their platform for coronavirus contact tracing, a key tool in trying to tame the pandemic.

"The work public health officials are doing around the world humbles us all," the two companies said in a statement.

"Google and Apple are clear that this is not a panacea but we do believe Exposure Notifications can make a contribution to the broader work of contact tracing," they said.

Under the notifications system, someone exposed to a person who tests positive for Covid-19 will receive an alert on their phone.

"Public health authorities will take the lead with this technology, and we will continue to support and advocate for it."

They said they would release software updates for health authorities to use to deploy their own apps, developed by their own technical staff.

In Europe, most countries are leaning toward use of the Apple-Google platform but France and Britain have opted to develop their own systems, currently being tested.

The two US firms said 22 countries had so far asked to use their platform and they expect more to come on board in the coming weeks.

Amid concerns about the security and use of the personal data such tracing apps will generate, Google and Apple laid down several conditions for the use of their technology.

The first is that any app based on it must be voluntary, not gather geolocation data and not be used for commercial purposes.

In addition, only one app per country is allowed so that there is no competition involved while it will be up to the individual user to declare if they have been infected with the virus or not.

When the crisis has disappeared, the system must be taken down.