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A prison in the US state of Minnesota was placed into lockdown on Sunday after dozens of inmates refused to return to their cells.

The protest, staged by around 100 prisoners, was later "resolved without incident", officials said.

Inmates were unhappy at being kept in their cells due to understaffing over Labor Day weekend, the state's Department of Corrections (DOC) said.

All inmates had now returned to their cells, a DOC spokesperson added.

Extra police units, firefighters and other emergency teams were stationed outside the facility in Baywater, about 25 miles (40km) east of the state's largest city Minneapolis.

Advocates for the inmates said the incident was a protest over prison conditions, including the excessive heat, limited access to showers and ice and unclean drinking water.

A US National Weather Service heat advisory is in place until Tuesday for the area, warning temperatures could reach up to 100F (37.7C).

The increasing frequency of dangerously hot conditions has drawn renewed attention to US prisons and calls for reform.

The DOC says the prison is short of 50 officers, which has been exacerbated by the holiday weekend, leading to intermittent lockdowns of inmates since Friday.

It means prisoners have been kept in their cells for longer periods, reportedly with no air conditioning.

The DOC blamed the unrest in part on the inmates' frustration over limited access to phones, recreation and showers, but refuted the claim that inmates were lacking access to clean water.

A spokesperson added that no one was hurt and the situation throughout the day was "calm, peaceful and stable".

DOC commissioner Paul Schnell said inmates were usually given several hours a day during the weekend for recreation, but holiday-related staff shortages had cut that down to just a single hour.

AFSCME Council 5, the union which represents Minnesota correctional officers, also said that understaffing was to blame for Sunday's incident.

A spokesperson said the incident was "endemic and highlights the truth behind the operations of the MN Department of Corrections with chronic understaffing".

Such conditions upset inmates because of restrictions placed on program and recreation time "when there are not enough security staff to protect the facility", they added.

In total, about 1,200 inmates are incarcerated at the facility, according to department records.