President Barack Obama Wednesday promised federal help for those hurt by deadly tornadoes in the southern US state of Arkansas, after inspecting damage on a brief stopover.

At least 36 people were killed when tornadoes tore through six central and southern states, including Arkansas, Alabama, Iowa, Mississippi, Oklahoma, and Tennessee last month.

"So the people of Vilonia and all the other towns devastated by the storm understand: there's a lot of work that remains to be done," said Obama standing amid the ruins of the Arkansas town, with local officials at his side.

"I'm here to remind them that they're not doing this work alone. You're country's going to be here for you," he said.

"We're going to support you every step of the way. You are in our thoughts and prayers. "

Obama has decreed a state of emergency for the affected states, in order to unblock federal aid.

With an average of 1,200 tornadoes a year, the United States is the world's most tornado-prone country. The twisters are particularly frequent in the great plains states -- Texas, Oklahoma and Kansas -- as well as in the southeastern state of Florida.

Obama's stop in Arkansas came the day after his administration published a massive report on climate change. According to the report, the effects of warming are already being felt, in particular by sparking weather events that are more violent than in the past.