LAS VEGAS, Nevada (CNN) -- A fire charred the top of two of three wings of the Monte Carlo Resort and Casino on Friday, causing no major injuries but forcing visitors and employees to evacuate, authorities said.

A bit over an hour after it began, the fire was fully contained, according to the fire departments in Las Vegas and Clark County.

Firefighters would remain at the hotel until "the fire is completely knocked out," said Clark County Fire Chief Steven M. Smith.

That would be at least through the afternoon, he said.

It was mainly an exterior fire, although there might have been slight damage to some rooms, he told CNN after a news conference.

The cause of the fire was unknown. Welders had been working on the roof.

No one was trapped, and there were no reports of major injuries, said Ed Cagalo of the Clark County Fire Department. The 32-floor building was completely evacuated.

The blaze was reported about 11 a.m.

Black smoke and flames poured from the fire, which roared through the building's exterior stucco and foam until firefighters were able to get on the roof and knock down the flames.

Most of the damage was to the top floor of the building. Falling debris ignited parts of an exterior ledge four floors below.

"High-rise fires are never easy to fight. As you can see with it being outside, our firefighters actually had to hang out the windows to try and cut the fire off. We directed our fire streams at an angle so we could make contact with the fire. ... It wasn't an easy fire at all," Smith said.

He urged motorists and pedestrians to avoid the area.

Earlier, the smoke was visible from more than a mile away, CNN producer Darian Billington said.

Billington said traffic was backed up on the Strip, and crowds of people were gathering to watch the blaze.

"It was horrible, there was fire everywhere," a hotel worker who watched the flames from a parking lot told CNN affiliate KVBC.

Four U.S. Air Force HH-60 Pave Hawk helicopters with rescue crews were put on standby at Nellis Air Force Base during the afternoon to assist in any rescue efforts, but were not needed, a Nellis spokeswoman said.

The hotel has more than 3,000 rooms, including 211 penthouse suites, and conference rooms.

The more expensive rooms are on the top floors.

The hotel was built in 1996 for $344 million. It is in the heart of the resort corridor.

The Monte Carlo, a subsidiary of MGM Mirage, has about 3,000 employees.

In 1980, a fire across the street at the MGM Grand Hotel, now Ballys, killed 84 people and injured nearly 700.