Eighteen guards and inmates at a jam-packed Philippine prison have tested positive for the coronavirus, officials said Friday, heightening fears of a rapid spread of the illness inside the country's jails.

Another 30 prisoners were showing symptoms inside the Quezon City Jail in Metro Manila -- a facility so crowded that inmates take turns sleeping on staircases and open-air basketball courts.

• 9 inmates, 9 personnel at QC jail test positive for COVID-19

The outbreak has fuelled calls from rights groups for the early release of prisoners charged with non-violent offences as well as the sick and elderly in an effort to ease congestion and lower the risk of transmission.

The Philippines has a steadily rising number of confirmed coronavirus cases, with 5,878 infections and 387 deaths as of Friday.

Social distancing is all but impossible in the prison system, where cells sometimes operate at five times' capacity.

• BJMP: Male detainee may have died from new coronavirus

Overcrowding has become an even greater problem since President Rodrigo Duterte launched a drug crackdown in 2016 that has seen thousands sent to jails.

Nine inmates and nine prison staff tested positive for the virus, Bureau of Jail Management and Penology spokesman Xavier Solda told reporters.

The prisoners were isolated and staff told to self-quarantine at home, Solda said.

"We are still in the process of intensive contact tracing," he added.

The Philippine Supreme Court on Friday deferred a decision on whether to release the most vulnerable prisoners, instead ordering the government to submit a report on measures it had taken to contain the virus inside jails.

"The release of prisoners and other measures to address the severe congestion in our jails is literally a matter of life and death," Free Legal Assistance Group chairman and Duterte critic Jose Manuel Diokno said Friday.

Otherwise the virus "will run amok", Diokno told AFP.

Human Rights Watch Deputy Asia Director Phil Robertson said Manila must "act urgently to mitigate what could be a catastrophe inside the country's overcrowded prisons"