The warning note was written in a scrawling handwriting and nailed to the town hall of the Mexican city of Ciudad Juarez that borders El Paso Texas in the United States.

"For those who don't believe it we will do it," it said, followed by the names of 17 police officers.

Just days later, four of those on the list were dead, ambushed by masked attackers firing automatic weapons and driving brand new GMC SUV's.

The murders of the Juarez officers are part of a recent wave of attacks on Mexico's law enforcement agents as Felipe Calderon, the Mexican president, tries to crack down on heavily-armed drug cartels.

Since January 1, assailants have killed more than 30 policemen, soldiers and judges across the country in highly-organised ambushes and assassinations.

Orchestrated attacks

In one killing, a group of armed men went to the house of a police officer in Tijuana and shot him dead, along with his wife and nine-year-old daughter.

Armed men have also battled police and soldiers in clashes lasting up to three hours, turning Mexican residential areas into war zones.

Analysts say the orchestrated attacks show that Mexico's longstanding drug violence has entered a new phase.
 

Now, the gangs are working together to defend these routes from the federal government.

"We used to see narcos against narcos. Now we are seeing narcos against cops," Jorge Chabat, a Mexican drug expert, told the newspapers using the common Mexican name for drug traffickers.

"Calderon is paying the price for taking on the cartels. As he has five years left in power, he might just get away with it."

US rewards

The president's offensive has included sending more than 25,000 soldiers and federal police after the gangs, extraditing alleged leaders to the US and making record drug busts.

In November, soldiers seized 23.5 tons of cocaine – enough for about 200 million lines of the drug – in the world's biggest ever cocaine bust. It was destroyed in a public bonfire.  

Calderon is hoping his crackdown will bring his administration rewards from the US, which has frequently urged its southern neighbour to be more aggressive against drug mafias.

The US Congress is debating a $1.4 billion anti-drug aid proposal for Mexico, including high-tech phone-tapping equipment and possibly Black Hawk helicopters.
 
The proposal has been tacked onto a bill requesting more funding for US military operations in Iraq and Afghanistan.

Calderon argues the US government has a responsibility to help out because US drug users fund the traffickers - analysts estimate that Mexican drug trade to the US is worth around $10 billion to $30 billion a year.

Most of the weapons fuelling the bloodshed are also bought in US gun stores and smuggled south over the Rio Grande.

"It's an extremely bloody uphill battle. Narco-organisations have a developed infrastructure. It is being threatened and we are seeing their brutal response."

Collusion fears
 
However, critics worry that Black Hawks or other such tools given to Mexico could fall into the wrong hands. 

In the last two decades, hundreds of police, soldiers and politicians have been convicted of working for the cartels.

One entire unit of army special forces deserted in the late 1990s to form a paramilitary commando called the Zetas, who work as bloody enforcers for the Gulf Cartel.

Their rival, the Sinaloa Cartel, imitated their paramilitary style by training hundreds of would be enforcers in special weapons and tactics.

After years of beheadings and reprisal massacres, these two cartels recently reached a truce, only to turn their wrath on the federal government, according to Mexican and US drug officials.

The level of firepower of the drug gangs has been shown in raids on cartel safe houses in recent weeks.

Police stormed one middle-class Mexico City home to find 30 guns, 12 grenade launchers, 30 grenades and more than 40 bullet-proof jackets with the initials FEDA – a Spanish acronym for "Special Forces of Arturo Beltran", an alleged drug gang leader.

A raid on a Tijuana warehouse used by a cartel for training even unearthed a shooting range and assault course.

Low ranking police officers complain they are outgunned and are risking their lives for salaries which are as low as $600 per month.

"How are we supposed to confront these guys if they come at us?" Osiel Mendoza, a Mexico City police officer, said to the media.
 
"We need the army to wipe them out."

Abuses criticism

Tijuana has seen several gun battles between authorities and drugs gangs [AFP]

However, Calderon's reliance on the military to take on the cartels has also sparked criticism.

In recent weeks, groups including Amnesty International, Human Rights Watch, and the United Nations have highlighted accusations of soldiers torturing, raping and killing civilians during their missions against drug gangs.

In one case, 19 soldiers face a court martial for shooting dead two women and three children in June at a roadblock in Sinaloa state.

In another incident, soldiers are accused of detaining and raping four girls in the mountainous state of Michoacan.

"We all need to reflect on the consequences of this type of activity and speak about security initiatives that comply with the rule of law," Louise Arbour, the UN high commissioner for human rights, said in Mexico City earlier this month.

"The Mexican army is not made for the fight against crime."