NEW YORK (AP) — Air traffic is coming to a standstill in much of the northeastern U.S. as a powerful storm brings heavy snow and strong winds across the region.

Thousands of flight disruptions piled up Monday morning. As of around 10 a.m. ET, more than 5,500 flights in or out of the U.S. were canceled, according to online tracker FlightAware. Hundreds of others were delayed.

New York's John F. Kennedy International Airport and LaGuardia Airport had the highest numbers of cancellations Monday — followed by airports in Boston, Newark, New Jersey, Philadelphia and Washington, D.C.

More than 4,000 U.S. flights were previously canceled Sunday. And another nearly 1,600 trips scheduled for Tuesday have already been cut, in disruptions that could continue to climb.

The Federal Aviation Administration urged travelers to monitor updates and check with their airlines on the status of their flights.

The National Weather Service on Monday called travel conditions “extremely treacherous” and “nearly impossible" in areas hit hardest by the storm.

Beyond air traffic, millions of people are stuck at home under road travel bans amid blizzard warnings. On Sunday night, cellphones across New York City received wailing push alerts announcing a ban on non-emergency travel on all streets through noon Monday, due to “dangerous blizzard conditions.” Rhode Island and New Jersey implemented similar restrictions.