New Zealand's prime minister said two people have been killed after a 7.8-magnitude earthquake struck Sunday just north of Christchurch — the city devastated by a deadly 2011 temblor.

Prime Minister John Key warned that the death toll could rise as rescue workers search isolated parts of the country.

"We've had very limited communications there because they've been cut off, so we've been working through police comms during the night," he said. "We also want to thank everyone for the response that's been going on."

In a statement, police confirmed that one person was killed when a building collapsed near the epicenter of the quake. It was unclear how the second person died.

Powerful aftershocks continued to rattle the region into Monday afternoon. The U.S. Geological Survey said a 6.2 magnitude quake struck 72 miles northeast of Christchurch at 12:34 p.m. (6:34 p.m. ET Sunday).

In a statement, New Zealand Police reported two additional casualties, including a possible death in Mt. Lyford, north of Christchurch.

Images as they come in of the #eqnz damage: Slip impact on State Highway 1 pic.twitter.com/KOpahqedg5 — MarlboroughEmergency (@MarlEmergency) November 13, 2016

Some bridges and highways remained closed, and a local district council said crews were working to restore power, sewage and water to several affected communities. Dramatic images released by the council showed steep hillsides collapsed onto roads.

Emergency authorities said late Monday afternoon that a debris dam that formed on on the Clarence River had breached, sending a large volume of water downstream toward Clarence, TV New Zealand reported. People were advised to stay well away from the river, it said.

Earlier, a tsunami was generated and the first waves hit the South Island's east coast, according to the civil defense agency. The agency later downgraded the tsunami threat, but marine and beach threats remained in effect.

The initial earthquake struck just after midnight Sunday, jolting residents awake and forcing them to evacuate the coast for higher ground. The earthquake was strong enough to split roads and crack bridges.

California native Brandon Perez, who was traveling in New Zealand, said that the quake shook the entire room of his hostel and that everyone quickly evacuated.

"It just kept going and going and wouldn't stop," he said. "I've experienced earthquakes before, but once it got to the point where the entire room was shaking and things were falling down, I started to get a little worried."

A tsunami is possible. Anyone near the east coast of the south island should move to high ground #eqnz pic.twitter.com/XO1etK1WYq — MCDEM (@NZcivildefence) November 13, 2016

The USGS said the quake, which struck about 57 miles northeast of Christchurch, was magnitude 7.8. Civil defense officials had earlier put the strength at 5.6, with a series of aftershocks.

Some minor damage was also reported in the capital, Wellington, more than 120 miles away. There were no immediate reports of major damage or injuries in Christchurch, but some social media users posted pictures of upturned furniture.

New Zealand sits on the Ring of Fire, an arc of seismic faults around the Pacific Ocean where earthquakes are common.

In 2011, 185 people died in a quake that flattened many historic buildings in Christchurch.