A hydrogen explosion occurred Monday morning at the quake-hit Fukushima No. 1 nuclear power plant’s troubled No. 3 reactor, injuring three workers, but the reactor’s container was not damaged, the government’s nuclear safety agency and the plant’s operator said.

The 11:01 a.m. incident came after a hydrogen explosion hit the No. 1 reactor at the same plant Saturday, and prompted the Nuclear and Industrial Safety Agency to urge residents within a 20-kilometer radius to take shelter inside buildings.

‘‘We judge that the possibility of a large amount of radioactive materials flying off from there is low,’’ Chief Cabinet Secretary Yukio Edano told a news conference, adding that the injection of seawater to cool down the No. 3 reactor is continuing.

Tokyo Electric Power Co (TEPCO), the plant’s operator, separately said three workers at the plant sustained bruises and that it had called an ambulance. It said it had all its workers go inside buildings but that the radiation level was as low as 20 micro sievert per hour at 11:44 a.m.

The blast followed a report by the power company to the government earlier in the day that the radiation level at the plant had again exceeded the legal limit and that pressure in the container of the No. 3 reactor had increased.

The Fukushima No. 1 nuclear plant has been shut down since a magnitude 9.0 quake struck northeastern and eastern Japan on Friday, but some of its reactors have lost their cooling functions, leading to brief rises in the radiation level over the weekend.

Since residents within the 20-km radius from the plant were asked Saturday to evacuate in the wake of the initial blast at the plant’s No. 1 reactor, there remain about 475 people in hospitals and nursing care facilities within the radius, plus several residents, according the agency.

The agency ruled out the possibility it will broaden the area subject to the evacuation order for now.

On Monday, radiation at the plant’s premises rose over the benchmark limit of 500 micro sievert per hour at two locations, measuring 751 micro sievert at the first location at 2:20 a.m. and 650 at the second at 2:40 a.m., according to the report.

The hourly amounts are more than half the 1,000 micro sievert to which people are usually exposed in one year.

The maximum level detected so far around the plant is 1,557.5 micro sievert logged Sunday.

The utility had been pouring seawater into the plant’s No. 1 and No. 3 reactors to help cool their cores, which are believed to have partially melted after part of the fuel rods were no longer covered by coolant water when levels fell following the quake.

The seawater injection stopped around 1 a.m. due to the shortage of water left in tanks, but resumed for the No. 3 reactor at 3:20 a.m., according to the nuclear safety agency.

The halt of coolant water injections apparently caused rising pressure in the reactor container and an increase in the radiation level at the plant, the agency said.

TEPCO at one point planned to release radioactive steam from the No. 3 reactor container to depressurize it and ordered workers to vacate the site. But as the pressure later lowered, workers resumed operations at the site, according to the agency.

Edano said pressure in the No. 1 reactor container has been stable and that seawater injections for the reactor will resume later.