Bahraini authorities have agreed to offer prisoners extra rights, including more visiting hours, following a hundreds-strong hunger strike that activists say is the largest in the country's history.

The interior ministry late Monday said it would "increase the duration of visitations" and was looking to increase the time inmates are allowed outdoors -- a step that has so far failed to quell the hunger strike at Jau prison that started in early August.

According to the Britain-based Bahrain Institute for Rights and Democracy (BIRD), at least 800 inmates are taking part, many of them dissidents detained during a 2011 crackdown on Shiite-led protests. Bahraini authorities have downplayed the incident.

"This offer is too little, too late. It comes after 22 days of Bahrain's biggest hunger strike in its prison history," said Sayed Alwadaei, BIRD's advocacy director.

"It is clear that the hunger strike will continue until the government addresses their concerns seriously and in good faith," he told AFP in a statement.

The strike has triggered rare street protests by relatives of inmates demanding their immediate release.

The head of Bahrain's National Institution for Human Rights met with prisoners at Jau over the weekend to hear their complaints, Alwadaei said.

In a statement published this month, the inmates said they were kept in their cells for 23 hours a day. They called for proper medical care, access to education and permission to pray together at a prison mosque.

"The international community needs to immediately step up their support for the hunger-striking prisoners in Bahrain," Niku Jafarnia, Bahrain researcher for Human Rights Watch, said this month.

The hunger-strikers include Abdulhadi al-Khawaja, a 62-year-old Bahraini-Danish human rights defender who has been imprisoned for 12 years.

He was rushed to the intensive care unit of a Bahraini military hospital with serious cardiac problems on August 11, only days after he joined the hunger strike, according to his daughter and rights groups.

"He continues to require urgent and adequate medical care, which prison authorities are failing to provide," said a letter sent to US President Joe Biden's administration this month by non-governmental organisations including HRW and Amnesty International.

Bahrain is a key regional ally of the United States and is home to the US Navy's Fifth Fleet.

It has imprisoned scores of dissidents since the uprisings of 2011, when authorities backed by a Saudi military force crushed Shiite-led protests demanding a constitutional monarchy and an elected prime minister.