NUEVO ORIZABA, Mexico — Transnational drug cartels are carving out new sanctuaries along this wild, neglected frontier with Guatemala, muscling their way into Central America’s weak states and moving freely over uncontrolled border crossings that do not appear on any map.

More than 90 percent of the South American cocaine that reaches the United States now funnels through Guatemala and across Mexico’s southern border, according to the latest intelligence from the U.S. Drug Enforcement Administration. The traffic is part of a vast, lucrative, dangerous trade in smuggled goods, from chickens and cheese to new AK-47s and aging hand grenades.

Secretary of State Hillary Clinton travels to Guatemala City on Wednesday to negotiate the region’s first cooperative security plan with the seven presidents of Central America. The Obama administration has promised to spend $200 million to help Central American countries fight the cartels and regain control of lawless territories.

But efforts to police the Mexico-Guatemala borderlands may be overmatched by its jungles, mountains and stubborn contraband culture.

On a dirt track outside this frontier town, only a handmade sign — “Mexico Starts Here” — marks the border with Guatemala. There are no soldiers, no customs, no passport controls — just rough truckers smuggling gasoline south and ragged migrants marching north.

The U.S. embassy in Mexico states that while there are 30,000 U.S customs and border patrol officers watching over the 1,926-mile U.S. border with Mexico, there were only 125 Mexican immigration officials monitoring the country’s 540-mile border with Guatemala.

A 2010 cable from U.S. diplomats, made public by Wikileaks, reveals that Guatemala has only a single helicopter and five pick-up trucks to patrol its entire border with Mexico, “a valuable buffer zone” to protect the United States.

On the Guatemalan side last month, the severed heads of 26 men and women were left scattered at a jungle ranch owned by a local trafficker who allegedly ran afoul of the Mexican cartel known as Los Zetas. The Guatemalan prosecutor investigating the crime was murdered, too, his decapitated torso left in a trash bag outside the governor’s house.

Guatemalan President Alvaro Colom last week extended an emergency “state of siege” in the northern border province of Peten, as the attorneys general of Mexico and Guatemala pledged for the first time to share intelligence and resources to beat back the Zetas, who are expanding into Central America.

Andres Granier, governor of the Mexican state of Tabasco, which shares the border with Guatemala, called the frontier “out of control” two weeks ago, begging the government to send soldiers and police.

Two reporters traveled 500 miles over the border’s roads and rivers last week. To call this boundary “porous” would be to suggest that parts of it are not. For the indigenous peoples, ranch hands and smugglers who traverse it freely, there is no border at all. It is a line on a map.

On the Suchiate River near the Pacific Coast, boatmen pole makeshift rafts through the currents like gondoliers, ferrying beans, gasoline, beer and diapers into Mexico or Guatemala in plain view of authorities. The trafficking is so well established that ferrymen from Mexico and Guatemala alternate work days on the river.

At a Mexican immigration station 50 miles north, a handful of sophisticated biometric machines donated by the U.S. government issue identification cards to Guatemalan maids and coffee pickers, scanning their irises and recording their thumbprints, while underneath the same bridge, porters stripped to their underwear wade through rapids with baskets of live chickens and tattered suitcases on their heads.

Dirt roads snake past Mayan ruins, thatched huts and cattle ranches slashed and burned from the tropical forest, and at night convoys of big SUVs carrying gunmen have the roads to themselves, as soldiers shut down checkpoints after the sun sets, according to U.S. statistics.

“This place is totally abandoned,” said Daniel Martinez, a town official in Nuevo Orizaba, standing outside his cantina, the Titanic, with no shirt and a wooden crucifix around his neck. “There’s no authority here. It’s a lawless place.”

A confidential cable sent by U.S. deputy ambassador John Feeley from Mexico City in 2010, released by Wikileaks, concluded “neither country presently works seriously to enforce laws” along “an extremely porous border” where U.S. officials “witnessed almost as many individuals crossing the border illegally as legally.”

In meetings with U.S. diplomats, Mexican President Felipe Calderon exhorted the U.S. to watch Guatemala and Belize since their internal weaknesses make them vulnerable, and he said he was worried about his southern border, according to U.S. officials.

Faced with the challenges of policing such a vast area, the Mexican government has opted to put military roadblocks north of the border along major highways through Mexico’s interior. But every choke point creates detours and opportunities for corruption.

Mexican authorities say their goal is to preserve the flow of commerce while intercepting drug shipments, weapons and criminals. At an army checkpoint south of the Mayan ruins at Palenque, colorful posters carried snapshots of the soldiers’ recent seizures — pistols, piles of bullets and assault rifles. There were a handful of plastic-wrapped packages of drugs, representing a small dent in the estimated 500 to 700 metric tons moving from South America into the United States each year, according to Justice Department statistics.

“Our policy isn’t to close the border,” said Mercedes Gomez Mont, Mexico’s top immigration official in the region. “It is a policy of flexibility.”

Establishing a U.S.-style, militarized border with fencing, cameras, heat sensors and agents would be a costly undertaking and would likely fail to end smuggling, given the dense jungles and mountain ranges. Geographic boundaries, such as the Usumacinta River, tend to facilitate the movement of goods and people as trafficking corridors, rather than act as barriers.

As the United States has helped tighten enforcement along smuggling routes at sea, Mexican cartels are now battling for control of land routes through Central America.

But U.S. officials concede that only about $20 million of the $1.6 billion Merida Initiative aid package to Mexico has been assigned to security for Mexico’s southern border. So far, the U.S. military has provided only night vision goggles, nine inflatable rafts and training for just 80 Mexican marines, according to the U.S. State Department.

U.S. military officials emphasize intelligence-gathering and winning “hearts and minds,” lessons they say they learned in Iraq and Afghanistan.

On the muddy Usumacinta River, Mexican soldiers stood watch at a cement quay where traders loaded sacks of beans into motorized canoes. On the Guatemala side, men pushed a sputtering truck loaded with corn up the river bank. No one inspected anything, and there was no sign of river patrols.

“There are two powers here: the soldiers and the criminals. We’re caught between them,” said Atilano Moreno, leader of a rural communal village at Santa Rita la Frontera, who can see Guatemala from his yard. He described the traffickers as if they were a natural part of the landscape, like the towering ceiba trees or the Zebu cattle. “These are the borderlands,” he said.