By Emily Le Coz

JACKSON, Miss. (Reuters) - Tornadoes unleashed by thunderstorms along the U.S. Gulf Coast ripped through southeastern Mississippi on Tuesday, killing four people, injuring many and causing extensive damage, authorities said.

One of the heaviest-hit areas was a commercial district along a U.S. Highway 98 bypass in the town of Columbia in Marion County, about 30 miles (48 km) west of Hattiesburg, state emergency management spokesman Greg Flynn told Reuters.

"We've got whole roofs lying in the road, people trapped in houses, cars flipped over," Marion County Sheriff Berkley Hall said.

One woman was killed in a shopping strip in Columbia, a rural community of about 6,500 people, and another in a nearby trailer home, according to county coroner Norma Williamson.

"It's chaos over here," she said of the aftermath of the storm, which struck at about 2:30 p.m.

Images of the scene published on the website of Hattiesburg television station WDAM showed mangled buildings, snapped trees, debris-littered roads and overturned vehicles, including an ambulance.

Two more storm-related fatalities were confirmed in Jones County to the northeast, where a separate tornado touched down an hour later, and both victims there were believed to have died in their homes, county emergency management spokeswoman Tammy Wells said.

Flynn said numerous people were hurt in both counties, some seriously. He described property damage in Marion County as "massive."

Governor Phil Bryant issued a state of emergency for Marion and Jones counties, hastening the availability of state resources needed for storm relief.

His office said the storms knocked out power to more than 7,000 customers.

The twisters were spawned by thunderstorms that originated over south-central Louisiana, then tracked northeast before barreling through southeastern Mississippi, said Corey Mead, a meteorologist for the national Storm Prediction Center in Norman, Oklahoma.

About 15 homes were damaged in southeastern Louisiana by a storm that also knocked down trees and power lines across Tangipahoa Parish and unleashed a tornado in the town of Amite, though no injuries were reported, according to the office of Governor Bobby Jindal.

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Mead said intensity of storms diminished after they passed through Mississippi, but dangerously high winds and additional tornadoes were possible late Tuesday and early Wednesday across parts of southern Alabama, northern Florida and southern Georgia.

Flash flood warnings also were posted across the Florida panhandle and parts of Georgia as a wave of heavy showers swept in behind the storm front, Mead said.

(Reporting by Emily Le Coz in Jackson, Miss.; Additional reporting and writing by Steve Gorman in Los Angeles; Editing by Eric Beech, Robert Birsel)