Italy should release more than 10,000 inmates from its overcrowded prisons, a watchdog group urged on Wednesday, as the government scrambles to fight the spread of coronavirus behind bars.

Ten prisoners have already tested positive for coronavirus, which can be transmitted through close contact with others, Italy's justice ministry said on Tuesday, before the government signed new measures to release some low-risk inmates to house arrest.

In a statement, the ministry sought to play down the spread of the virus in announcing the latest case in a prison in Voghera, in the northern region of Lombardy hardest hit by the virus that has killed 2,503 people throughout the country and infected 31,506.

"At the moment these are isolated episodes and there is no evidence of infection caused by these positive cases," the ministry said.

But prisoner rights group Antigone said the government measures were insufficient given severe overcrowding in the system, which forces inmates to share cells and come into close proximity with each other.

"Prisons are in danger of becoming a health bomb that can have repercussions on the very integrity of the national health system," Antigone's president Patrizio Gonnella said, adding that recently announced new measures were "clearly insufficient".

The group said more than 14,000 inmates should be released to relieve overcrowding, which, if unchecked, will exacerbate the spread of the virus.

Prisoners rioted earlier this month in 27 institutions across Italy over coronavirus fears and the suspension of family visits
Prisoners rioted earlier this month in 27 institutions across Italy over coronavirus fears and the suspension of family visits AFP / Miguel MEDINA

A new government decree that came into effect on Wednesday said certain categories of inmates serving sentences of less than 18 months would be allowed to serve them under house arrest, to be monitored with electronic bracelets.

Exceptions to the new regulations, which may last until June 30, include violent and habitual offenders, and inmates associated with organised crime.

Prisoners who took part earlier this month in riots in 27 institutions across Italy will also be unable to benefit from the measure. The riots were sparked by inmates' worries over coronavirus and the suspension of family visits.

Thirteen inmates died in the rioting, according to the government, most through drug overdoses after prison infirmaries were raided.

The pool of a maximum of 3,000 inmates targeted by the government as eligible for house arrest falls far short of the over 10,000 needed to reduce overcrowding, Antigone said.

Some 61,230 people are behind bars in Italy, 20 percent more than the maximum capacity of 50,931, according to prison administration data.

Some prisons in the country are at 200 percent capacity, often resulting in three inmates sharing cells of just 12 square meters (130 square feet) or less.

In Voghera, the infected prisoner is being treated in the city hospital and his cellmates were quarantined for 14 days, while the affected wing of the prison is isolated, the justice ministry said.

Two other infected inmates are in quarantine inside special rooms for overnight stays within their prisons, while the rest are in local hospitals.

Some contracted the coronavirus outside prison while hospitalised, the ministry said.