LONDON - British Prime Minister David Cameron sent hundreds more troops into northern England on Sunday to help exhausted residents and emergency workers fight back rising river waters that have inundated towns and cities after weeks of heavy rain.

Cameron called the flooding "unprecedented" and vowed to do everything possible to protect people and their property as the damage spread to the major cities of York, Leeds, and Manchester.

He said protective systems and contingency plans will be reviewed because the frequency of such extreme weather events seems to be on the rise.

Persistent rainfall over several weeks has saturated the ground and swollen the rivers to record levels, leaving entire swathes of northern England, and smaller parts of Wales and Scotland, vulnerable.

Several hundred flood warnings remain in effect.

There have been no fatalities or serious injuries reported, but hundreds of people have been evacuated from houses and apartments and a hotel in York, 200 miles north of London, where 3,500 properties are judged to be at risk.

Emergency crews worked extra shifts to try to restore power to roughly 7,500 blacked-out homes in the greater Manchester and Lancashire areas.

Environment Secretary Liz Truss said flood protection systems put in place in recent years were unable to cope with the record-high river levels.

"In Lancashire, every single river was at a record high," she said. "In Yorkshire we have seen some rivers a meter [yard] higher than they have ever been before.

"Clearly, in the light of that, we will be reviewing our flood defenses," she said

Several hundred people had been evacuated the day before in the West Yorkshire and Lancashire regions and officials said thousands had lost power. The number of people affected continues to grow as flooding spreads and impacts cities as well as villages and towns.

A picturesque 200-year-old pub, the Waterside, in the greater Manchester area, collapsed and part of the structure was swept away by the Irwell River. Rising river levels also threatened downtown Manchester and police dealt with a ruptured gas main and small fire believed to have been caused by the flooding.

The Environment Agency urged residents to remain vigilant because more "severe" flooding was expected overnight and Monday.

The agency used its Twitter feed to beg people to be cautious, tweeting that driving or walking in flowing floodwaters could be life-threatening.

Rescue crews have been using boats to help remove people from their homes and, in some cases, from their nearly submerged vehicles.

"We are now entering the recovery phase of the operation and a lot of work is being done to return Greater Manchester to normality," Assistant Chief Constable John O'Hare said, according to Bloomberg News. "There is still a lot of work to be done in some areas so this phase may take some time."