CIUDAD JUÁREZ, Mexico – An explosion of violence has made this border city the central front in Mexico's war on powerful drug cartels. But officials disagree on whether the carnage is a sign of progress or a mark of failure.

Nearly five months after the government of President Felipe Calderón sent more than 2,500 federal troops and police to the state of Chihuahua on Texas' southwestern border, the pace and scope of drug violence have reached unprecedented levels.

With two mass killings in recent days, more than 1,200 people have been killed statewide in drug-related violence this year, according to tallies kept by Mexican media.

Juárez is leading the country in killings, surpassing even Mexico City. More than 850 have been killed in this city of 1.5 million people right across the border from El Paso, including about 150 in the past three weeks.

The state toll makes up more than 40 percent of the nationwide total of 2,700 people killed in drug-related violence this year.

Chihuahua Gov. José Reyes Baeza this week called the violence "unacceptable" and challenged federal authorities to do more, saying that "a new response is needed to the bloody expressions of organized crime."

But a U.S. law enforcement official said the level of violence was a sign of success. "We're winning. The cartels are in disarray," the official said, speaking on condition of anonymity.

Others aren't so optimistic.

"What we're witnessing today is, without question, without precedent," said Alfredo Quijano, editor of Norte de Ciudad Juárez newspaper. "This is the fiercest, crudest war we've ever witnessed between drug traffickers in our city's history."

The carnage raises questions about the government's ability to restore order – particularly in cities such as Ciudad Juárez, an important trade and immigration conduit to Texas.

"These killings clearly indicate that the Mexican state has absolutely failed in its mission to protect the public," said Howard Campbell, a border anthropologist and drug trafficking expert at the University of Texas at El Paso. "It appears that no amount of soldiers or special police agents is enough to stop the killings in Juárez, either because they are incompetent, badly corrupted or intimidated by the cartels."

Officials are concerned that the violence is spreading beyond cartel and law enforcement targets. Both UTEP and New Mexico State University have warned students to stay away from Juárez, a traditional destination for partygoers.

U.S. and Mexican officials had predicted that 2008 would be a bloody year, partly in response to the government crackdown. But officials now say they are surprised by the ferocity and resilience shown by the warring Sinaloa and Juárez cartels, battling for control of smuggling routes into the U.S. And they expressed renewed concern of corruption within the government.

Last week, the federal attorney general's office put six members of its organized crime unit under house arrest on suspicion of working for drug kingpins, including Arturo Beltrán Leyva, a reputed leader of a faction involved in the Chihuahua violence.

Law enforcement officials attribute much of the violence to the Zetas, the paramilitary group once allied with the Gulf cartel but now operating more as freelance mercenaries. Officials say the Zetas are now working for Vicente Carrillo Fuentes of the Juárez cartel and his ally Mr. Beltrán Leyva, an alleged former top member of the Sinaloa cartel.

"You're talking about a lethal combination – brains and power," said a U.S. intelligence official, speaking on condition of anonymity. He said he was referring to Mr. Beltrán Leyva and his ability to co-opt Mexican authorities, and the muscle of the Zetas, a group that was founded by army deserters and had formerly served as the armed enforcers of the Gulf cartel along the Texas-Mexico border in Tamaulipas state. The Zetas have been implicated in Dallas killings as well.

Officials say the Zetas are increasingly involved in other criminal enterprises, including kidnapping and human smuggling, as well as legitimate businesses.

Even relatively quiet areas are sources of distress to some people along the border.

Nuevo Laredo has seen a sharp drop in violence after hundreds of soldiers and federal agents were dispatched there. But business leaders say the Zetas maintain an active presence and are pressuring their way into legitimate businesses.

"This is worse than when they were fighting it out because at least they were out in the open," said Jacob Suneson, former vice chairman of the Nuevo Laredo Chamber of Commerce. "Now they're in the fiber of the city, kidnapping people at will, and nothing's said or published anymore, nothing is reported, nothing is being done. If our federal government says it's winning the war and that it's in control, I would ask them the following: How are you measuring success? Because from our vantage point, we've lost Nuevo Laredo."

Interior Minister Juan Camilo Mouriño said Wednesday that the government remains committed to restoring order.

"The president does not elude his responsibility and, on the contrary, will continue facing the most deeply felt complaint of society, which is insecurity, until the last day of his administration," Mr. Mouriño said. "This is a transcendent moment for the country."

Earlier this year, the Zetas and the Gulf cartel amicably broke their working relationship after dividing up areas of control in Tamaulipas, U.S. and Mexican law enforcement officials say. Under their agreement, the Gulf cartel controls smuggling at key border crossings, including Matamoros, Reynosa and Tampico. The Zetas control Nuevo Laredo, the busiest transit route into the U.S. via the Interstate 35 corridor to Dallas and beyond.

One Laredo law enforcement official said it was inevitable that the Zetas would return to challenge their former employers for wider control.

"Nuevo Laredo, the entire state of Tamaulipas, is a powder keg," said the official, citing informants. "Once the Zetas complete their mission with Beltrán Leyva, they will return and cause mayhem here. It's just a matter of time."

Staff writer Laurence Iliff in Mexico City contributed to this report.

Recent killings in Ciudad Juárez and Chihuahua state include:

Monday: Gunmen ripped off security bars and stormed a house in Juárez, killing a Mexican motocross champion, Rene Tercero Reyes Aguirre, and two other racers, the El Paso Times reported.

Saturday: Gunmen killed 14 people at a family gathering in the western town of Creel.

Aug. 13: Gunmen opened fire during a religious service at a drug rehabilitation center in Juárez, killing eight men.

Aug. 11: There were 15 killings in Juárez, including two quadruple homicides. In Chihuahua City, assassins killed Pedro Aragonez, the forensic science director of the state attorney general's office.