NEW YORK (AFP) - Two inmates convicted of murder escaped from New York state's biggest maximum security prison by cutting through cell walls, prompting a major manhunt, authorities said on Saturday.

Richard Matt and David Sweat sneaked out of the all-male Clinton Correctional Facility in Dannemora, New York State Police said in an alert.

Hours later, Governor Andrew Cuomo announced on Twitter that he was cancelling a scheduled appearance at the Belmont Stakes horse race to instead travel to the prison.

"These are two dangerous individuals, one was incarcerated for killing a sheriff, so these are dangerous people," he said during a briefing there.

"And they are nothing to be trifled with." More than 200 law enforcement officers were deployed in the search, assisted by a variety of tactical support, including K-9 units, SWAT teams and helicopters.

On Twitter, Mr Cuomo posted images of the path the men were believed to have cut to escape. He explained that the men had used power tools to drill through walls and pipes.

Matt and Sweat left sweatshirts and other clothing in their beds as decoys to make it appear that they were still in their cell.

No one has ever escaped from the maximum security portion of the prison, which opened in 1845, according to Cuomo.

"By definition, it was an extraordinary act," Cuomo said.

Matt, 48, and Sweat, 34, were found missing during a 5.30am (5.30pm Singapore time) bed check, according to state police.

"Both are considered to be a danger to the public. If located DO NOT approach them. Contact 911 or the New York State Police immediately," authorities said in a statement.

It was unclear whether the men were armed or had received help in plotting their escape.

Sweat was serving a sentence of life without parole for killing a sheriff's deputy, while Matt was serving a sentence of 25 years to life for kidnapping a man and beating him to death.

About 3,000 inmates live at the prison, according to the Village of Dannemora's website.