Heavy snow and strong winds battered the northeastern United States Thursday, intensifying post-Christmas travel woes, after violent weather left at least nine people dead and many without power.

More than 2,750 flights have been cancelled since Tuesday as the huge storm wreaked havoc from the Gulf of Mexico to the Great Lakes before heading for the northeast, including 500 on Thursday, according to flight tracker FlightAware.

In New York, high winds sparked major delays, but only 65 flights were outright cancelled at the three major area airports -- Kennedy, LaGuardia and Newark.

Winter storm warnings were in effect for parts of New York, New Jersey, Pennsylvania and New England, the National Weather Service said. Additional coastal flood warnings were issued from New York's Long Island to Maine.

So far, the heaviest snowfall was recorded in northern New York, with more than a foot (30 centimeters) on the ground in Rochester on the shores of Lake Ontario.

The storm was expected to continue to move to the northeast, dumping up to another six to 12 inches of snow on northern New York and New England.

"Winter weather has arrived for many parts of the state, so as a precaution we have opened the emergency operation center to coordinate response efforts using all state and local resources," New York governor Andrew Cuomo said.

And New York Mayor Michael Bloomberg used residents still without power two months after deadly superstorm Sandy slammed into the city in October to seek refuge in emergency shelters.

In Canada, dozens of planes were grounded in Toronto, Ottawa and Montreal due to the wintry weather, with 18 inches of snow expected in the southern part of Quebec province.

Earlier in the week, nearly three dozen tornadoes were reported in the southern US states of Texas, Louisiana, Alabama and Mississippi.

More than 200,000 people lost power in Arkansas, Mississippi and Texas, and Entergy, the regional utility company, warned Wednesday that it could take crews up to a week to restore electricity in all areas.

The governors of both Mississippi and Alabama have declared states of emergency.

In Indiana, authorities dispatched nearly 600 trucks to clear highways and smaller state roads, and advised travelers to stay home if possible or else leave themselves extra time.

The violent storms have so far claimed at least nine lives -- three in Arkansas, two in Oklahoma, two in Indiana and one each in Louisiana and Texas.