Water being delivered to Nevada's Owyhee, Rock Creek, and Little Humboldt herd management areas.
The muster, in the Owyhee, Rock Creek and Little Humboldt herd management areas, began on Saturday, but was called off after seven horses died the next day from the effects of dehydration. Another four have since died from similar complications.
The bureau said this week that, with drought conditions escalating, it had taken emergency action to provide water to wild horses within the gather area, which encompasses the Owyhee, Rock Creek, and Little Humboldt herd management areas.
It hoped the water would stablilise the condition of the horses and enable them to be captured. It said the animals were at risk of dying from a lack of rangeland water unless they were gathered.
The bureau muster contractor made an aerial flyover of the Owyhee area on Tuesday morning and located two large groups of wild horses - the Dry Creek group of about 125 wild horses; and the Star Ridge group of about 400 animals.
The Star Ridge group is around a dry reservoir and is making no attempt to move to the nearest water source, about 15 kilometres away at the South Fork Owyhee River.
The animals are considered at risk of dying from complications of water starvation/dehydration, the bureau said.
It said while some of the Dry Creek group had recently taken water, the bureau remained concerned and would continue to monitor the group.
"BLM Nevada considers this an emergency situation," said Ron Wenker, the bureau's Nevada state director.
"Yesterday [Wednesday] afternoon, the bureau implemented a short-term emergency strategy to provide the animals water during the next four to five days in an effort to stabilise their health condition."
It installed six water troughs with a combined capacity of 3000 gallons near and around a reservoir located about five kilometres from where the Star Ridge group is located, and used a water tanker to fill the troughs.
If the wild horses do not independently travel to the water, the bureau will assess having the muster contractor use his helicopter to gently and slowly guide the animals toward the water with the hope that they will drink.
"A large number of these excess wild horses could die if they are not gathered," Wenker added.
"Therefore, BLM Nevada's goal is to gather these animals within four to five days after stabilising their immediate water-starved condition."
He said the bureau could not sustain this level of support to provide water to the animals over the long-term because of the limited road access and the poor condition of roads in the area.
The bureau has organised a review team consisting of BLM and independent experts to analyse the on-the-ground conditions of wild horses and the public rangelands.
The bureau said it continued to care for the 216 wild horses in the on-site temporary holding corrals, following the initial muster last Saturday.
The health conditions of the animals appeared to be stabilising.
It shipped three truckloads with 88 mares and 41 foals to the Palomino Valley Centre regional adoption facility on Wednesday.