With rising river levels in Fort McMurray, Alta., threatening homes and businesses in the heart of the community's downtown, the mayor is asking for military assistance to limit the damage.

Flooding caused by ice jams in the rapidly thawing Athabasca River worsened overnight Sunday, prompting a new round of mandatory evacuation orders, a decision to block access to the downtown area, and a boil water advisory.

All of the community's lower townsite is now under a mandatory evacuation order with the exception of the Northern Lights Regional Health Centre.

The Grayling Terrace neighbourhood has been issued a voluntary evacuation notice.

Help has been requested from Canada's Armed Forces and the federal government to deal with "this new crisis" in the northern Alberta community, Don Scott, mayor of the Regional Municipality of Wood Buffalo, said in a post on social media.

Alberta Environment Minister Jason Nixon was scheduled to meet with Scott on Monday to assess what the community needs, a spokesperson for the provincial government told CBC News.

Premier Jason Kenney visited the community Monday afternoon, to offer support and survey the damage.

WATCH | Fort McMurray flooding forces evacuation, call for military assistance:

Flooding from spring thaw has forced the evacuation of downtown Fort McMurray, Alta., and prompted the municipality to ask Ottawa for military help. 1:59

"Fort McMurray has seen, of course, ice packs and spring floods in the past, and most recently a significant one 20 years ago," Kenney said. "But this apparently is a once-in-a-century situation. We took a flight over the city and saw the impact. It's very significant."

Kenney specifically addressed the idea that the Canadian Air Force could bomb the ice jams, a method he said had been used on a smaller ice pack in the 1970s.

"Seeking to bomb that length of ice pack, 25 kilometres, would not actually dislodge the ice, it wouldn't displace it, it would simply probably jam it up even further," said Kenney.

The premier, who was travelling with Paul Wynnyk, deputy minister of Municipal Affairs, said he wants to ensure all necessary support is being provided in response to the local emergency.

During his visit, Kenney met with Scott, local MLAs Tany Yao and Laila Goodridge, and emergency management officials.

'No entry is allowed'

Early Monday morning, evacuation orders were issued for the lower townsite, including parts of Franklin Avenue, a main thoroughfare that runs parallel to the Clearwater River, one of three rivers running through Fort McMurray.

Streets were underwater and some residents could be seen evacuating through the flooded roads by boat.

"Access to the lower townsite, including MacDonald Island, downtown and Waterways, is temporarily closed due to flooding concerns," reads a municipal advisory issued shortly before 5 a.m. by the Regional Municipality of Wood Buffalo.

"No entry is allowed. Residents who leave the lower townsite will not be allowed re-entry."

This map issued by the Regional Municipality of Wood Buffalo at 11 a.m. Monday illustrates the areas of the lower townsite under mandatory evacuation order. (RMWB)

Flooding in the region began on Sunday as ice jams in the Athabasca forced water to move in the opposite direction and eventually caused water levels to rise in the Athabasca, Snye and Clearwater rivers.

The situation changed rapidly overnight. Between midnight and 6 a.m. Monday, the municipality issued six new emergency advisories to residents

The expanded evacuation orders issued Monday morning included specific roadways in the lower townsite: Demers Drive, Armit Crescent, Fitzsimmons Avenue and Father Mercredi Street. The Platinum Hotel on Franklin Avenue is specifically named.

Residents on Pond Crescent, Poplar Crescent and the southern portion of Alberta Drive closest to Hospital Street were also told to move to higher ground. People with homes from Riedel Street to Marshall Street along Manning Avenue also received notice.

By 1 p.m., mandatory evacuation orders stretched across much of the lower townsite with residential streets adjacent to Highway 63, nearly 10 blocks from Clearwater, being cleared of residents.

Fort McMurray resident Joseph Enverga took to the submerged streets to capture the extent of the flooding. 0:50

On Sunday, evacuation orders were issued for the Taiga Nova Eco-Industrial Park, Longboat Landing and Draper, an unincorporated community with less than 200 people located about 12 kilometres south of Fort McMurray.

Waterways, a neighbourhood south of downtown, was also given notice to evacuate.

Boil water advisory, state of emergency

Due to reports of discolouration in tap water of Fort McMurray neighbourhoods north of the Athabasca River, Alberta Health Services has directed a boil water advisory to be issued.

Water in the affected area is safe for bathing but should not be consumed by people or pets.

Areas south of the Athabasca River are not affected at this time, municipal officials said.

The RMWB declared a secondary state of local emergency on Sunday as rising water levels along the Athabasca, Snye and Clearwater rivers created a high risk of flash flooding.

A local state of emergency had previously been declared due to the COVID-19 pandemic.

Mandatory evacuations and a secondary state of local emergency were declared in Wood Buffalo on Sunday after rising water levels created a high risk of flash flooding. (Jamie Malbeuf/CBC)

Remain alert, residents warned

The Athabasca continues to break and water levels remain extremely high, the municipality warned on Monday.

Residents in flood-prone areas should prepare a 72-hour emergency kit and stay alert as the risk of further flooding remains.

"Every year the ice breaks and we have some sort of event. Usually, the ice flows on by. Once approximately every 20 years we have something different happen," Scott said in a phone interview Sunday.

"This happens to be that year."

Scott said officials had been monitoring an ice jam about 10 kilometres upstream, which let loose early Sunday morning.

Premier Jason Kenney visited Fort McMurray to see the scope of the flooding and meet with Mayor Don Scott. (Jamie Malbeuf/CBC)

Joseph Enverga, soup kitchen director for the Baptist Fellowship Church, was startled awake around 2 a.m. when the power went out. He quickly realized the streets around his downtown apartment were submerged in knee-deep water.

He was loading up his van when he was offered a ride by a man navigating the flooded streets in a boat.

Enverga, and the cat he was pet-sitting for a friend, were happy for the ride, he said.

"It was a crazy, crazy sight," he said. "It's such a sad picture of where our city is right now. We're trying to help people and get back on our feet.

"I was here for the fire and I watched the city kind of rebuild through that and then COVID hit … now, here comes this big flood. I appreciate the city and the resiliency of our people, but it's a lot."

'Be as neighbourly as possible'

Dan Edwards, executive director of the Wood Buffalo Food Bank, said the building, about two blocks from the banks of the Clearwater River, began flooding quickly on Sunday.

He was watching the rising water on the building's security cameras. Around 2 a.m., he rushed to the property in an attempt to save what he could.

Within the hour, as the waters threatened to submerge his vehicle, he was forced to leave.

When Edwards left, the office and adjoining warehouse were filled with at least six inches of water. With the power now out, he can't put an estimate on the damage. He suspects some food will be lost, either by coming into contact with water or by spoiling in freezers without power.

He said the food bank was already grappling with a 30 per cent increase in calls for service due to job losses caused by the pandemic and gutted oil prices. He expects the flood damage will only increase demand.

Edwards doesn't know if the food bank can continue to serve its clients during the flood or how quickly they will be able to resume operations, even if a temporary location were found.

Operations must be approved by Alberta Health Services.

"We're just going to have to play it by ear, and we're only a few hours into this day and knowing what the outcome is going to be, so we really are just asking everyone to have patience with us," Edwards said.

"Be as neighbourly as possible, and if you know someone that is struggling and you can help them out a little bit, that's going to go a long way right now."

Skateboard park looks like a 'brand-new swimming pool'

Phil Meagher, a municipal councillor who surveyed the situation by bicycle, said he could see a gravel truck that was almost completely submerged at an area of town called the Snye, where the Snye and Clearwater rivers meet.

"Everything else in the Snye is totally underwater. Even our skateboard park looks like a brand-new swimming pool," Meagher said.

Phil Meagher poses next to a huge ice chunk on the Athabasca River near Fort McMurray on Sunday. (Phil Meagher/The Canadian Press)

Evacuees were being directed to a drive-thru reception centre that has been set up at the Oil Sands Discovery Centre. They are being housed in hotel rooms that had been pre-booked by the municipality.

Scott said physical distancing recommendations to prevent the spread of COVID-19 are being followed.

He noted the municipality prepares every year for the possibility of flooding when the ice breaks up. When officials issued public health orders related to COVID-19 more than a month ago, Scott said they knew they'd have to incorporate the pandemic into flood planning.

Scott noted Fort McMurray's population is ready for disasters, such as the wildfire in 2016 that forced the entire city to evacuate and destroyed 2,400 homes and buildings.

"Because of the oilsands, people are very used to being in a very safety-driven environment," he said.

"And if there's any place that can face challenges, it's ours."