* Drug violence breaks out in once quiet state
* State government want more troops
* Army stretched in flashpoints across Mexico
By Robin Emmott
DURANGO, Mexico, May 20 (Reuters) - Drug gangs have forced open a bloody new front in Mexico's drugs war, extending their battles over smuggling routes into a formerly quiet northwestern state and further stretching the army.
A fight for control of the mountainous state of Durango has killed some 235 people this year, a jump in violence that poses a new challenge to troops already struggling to contain bloodshed along the U.S. border.
With only a few hundred soldiers in Durango, drug hitmen from eastern Mexico are taking over towns, kidnapping police, shooting up local government offices and slaughtering rivals.
It poses a fresh threat to President Felipe Calderon, who has staked his reputation on pushing back the cartels, and could fuel U.S. concerns that violence is overwhelming its southern neighbor.
The drug war has become the biggest challenge of Calderon's presidency and has started to spill over some parts of the U.S. border. Murders are rife in Mexican border cities like Ciudad Juarez, where Calderon sent 10,000 troops in March to tackle the gangs.
The outbreak of violence in Durango also marks a new challenge to top drug fugitive Joaquin "Shorty" Guzman on a formerly quiet patch of his home turf in northwestern Mexico.
Officials and analysts say Guzman, who has long been battling rivals in other parts of the country, is being attacked in Durango by the Gulf cartel and its brutal "Zetas" armed wing as they fan out from their base in northeastern Mexico, near Texas.
"We have a 600 percent increase in violence in Durango this year ... and we do not have the resources to attack these groups," the state's deputy attorney general, Noel Diaz, said in the state capital, also called Durango.
Durango state is part of Mexico's "Golden Triangle", a remote marijuana and opium-producing region that includes the Pacific state of Sinaloa, and Chihuahua south of Texas, through which gangs smuggle cocaine and other illegal drugs.
Sinaloa and Chihuahua have become two of Mexico's most violent states since Calderon launched an army assault on drug cartels in late 2006. Durango, home to Canadian-owned gold and silver mining projects, had been quiet, but some mining concessions are now too dangerous to explore, local mayors say.
"There is a long list of police killed and official buildings raided and attacked with grenades, with AK47s and burnt down," state secretary Oliverio Reza, the No. 2 in the Durango government, told Reuters. "We need an increase in troops more than ever."
Guzman, 52, has dominated smuggling in Durango for years and wed his 18-year-old bride in the state in 2007 in a ceremony ringed by armed guards.
Although Mexican officials deny he is living in the state, Durango's archbishop caused a storm in April when he said that "everyone except the authorities," knew Guzman was there.
Locals say Durango's colonial capital is no longer the genteel place it once was. "We're getting used to the sounds of gun battles, sirens and helicopters," said Dora Gonzalez as police cordoned off her street after an attack on an officer.
Across Mexico, drug violence has killed 2,300 people so far this year, apace with the 6,300 people murdered in all of 2008. The bloodshed is scaring investors, worrying some tourists and the central bank says it has hurt the peso currency.
Calderon, who has deployed 45,000 troops against the well-armed drug gangs, says he is winning the fight and the increased violence shows the cartels are under pressure. U.S. President Barack Obama has called his efforts "courageous".
But the killings in Durango may be a sign of how hard it is to crush drug gangs as military pressure in one region drives organized crime into less guarded areas. Durango's Reza calls it "the cockroach effect."
In Ciudad Juarez, on the border with Texas, a surge of troops cut drug killings by 80 percent, but the murder rate is now creeping back up again.
Durango, a sleepy state founded by the Spanish for its mineral wealth in the 16th century, is struggling to win more army support despite pleas from the state governor.
"Durango has been left on standby. There isn't the will to carry out that kind of military deployment in this region," said analyst Rafael Herrera, a former editor of a regional newspaper. (For more stories, photos and video on the drug war, click here) (Editing by Catherine Bremer and Kieran Murray)