By Alan Baldwin

BAKU (Reuters) - Formula One championship leader Nico Rosberg and McLaren's Jenson Button expressed concern on Thursday about the pit lane entry and lack of run-off areas at the new Baku street circuit.

Azerbaijan is hosting its first grand prix this weekend and the layout mixes the sport's longest straight, a 2.1km blast where cars reach around 340kph, and tight corners around medieval walls.

The pit lane entry involves a tight chicane separate from the main straight, but at unrestricted speed, with another long stretch before the driver finally has to engage the car's speed limiter.

Where the track wends around the old city, buildings on either side leave little room for error.

Run-off areas, s

Sydbwhere drivers can safely leave the track, provide space in the event of a mechanical failure or to correct mistakes without hitting anything.

"The track looks great, the place is great. A little bit of a concern with those run-offs. There are two or three which are really not looking good. That’s not great," Mercedes driver Rosberg told reporters.

"I really trust the (governing) FIA to get the job done, they have all the calculations and simulations. They know exactly the speeds," added the German.

"There are given rules they have to make tracks, so I would hope that they stuck to them. I’m doubting it a little bit, looking at those corners."

Red Bull's Australian Daniel Ricciardo said a driver could lose half a second on the pit lane entry if he slowed too early but there were also big risks in going too fast. Rosberg and Button, speaking separately agreed.

"There is a massive accident to be had," warned the German.

"It is like we have gone backwards in certain areas, I don't know why," said Button, a director of the Grand Prix Driver's Association (GPDA) and 2009 world champion as well as the sport's most experienced racer.

"A lot of the circuit is good...I think it is exciting in some places, but there are just some things that look like they need to be addressed."

Reigning world champion Lewis Hamilton, Rosberg's team mate, and Ferrari's Sebastian Vettel were less concerned about the layout.

"(The) pit entry looks a challenge -- but then again, I think that’s why we’re here," said four times world champion Vettel. "And I certainly have the plan to stay on track and not use the run-off."

(Reporting by Alan Baldwin; Editing by Toby Chopra)