(CNN) It was the second deadliest conflict in the world last year, but it hardly registered in the international headlines.

As Syria, Iraq and Afghanistan dominated the news agenda, Mexico's drug wars claimed 23,000 lives during 2016 -- second only to Syria, where 50,000 people died as a result of the civil war.

"This is all the more surprising, considering that the conflict deaths [in Mexico] are nearly all attributable to small arms," said John Chipman, chief executive and director-general of the International Institute for Strategic Studies (IISS), which issued its annual survey of armed conflict on Tuesday.

"The wars in Iraq and Afghanistan claimed 17,000 and 16,000 lives respectively in 2016, although in lethality they were surpassed by conflicts in Mexico and Central America, which have received much less attention from the media and the international community," said Anastasia Voronkova, the editor of the survey.

In comparison, there were 17,000 conflict deaths in Mexico in 2015 and 15,000 in 2014 according to the IISS.

The Mexican government lashed out at the report's writers. In a statement posted to its website, the government criticizes the report's characterization of Mexico having a non-international armed conflict, saying the military's policing of criminal gangs does not equate to what goes on in other countries. It also disagreed with the report's methodology.

The statement, from Mexico's interior ministry and foreign ministry, questions the number of killings in the report.

"The total estimate of intentional homicides at the national level in 2016 has still not been published by the National Institute of Statistics and Geography (INEGI), so it's unknown where the figure used in the report came from," the ministries said, according to a CNN translation.

There are other reasons for killings besides connections to drug gangs, the government said.

"In this sense, the report starts from a base that is erroneous and lacking in technical rigor," the statement said, adding that when figures are adjusted for population, many other countries are more violent than Mexico.

Rising death toll

Voronkova said the number of homicides rose in 22 of Mexico's 32 states during 2016 and the rivalries between cartels increased in violence.

"It is noteworthy that the largest rises in fatalities were registered in states that were key battlegrounds for control between competing, increasingly fragmented cartels," she said.

"The violence grew worse as the cartels expanded the territorial reach of their campaigns, seeking to 'cleanse' areas of rivals in their efforts to secure a monopoly on drug-trafficking routes and other criminal assets."

Mexican drug cartels take in between $19 billion and $29 billion annually from US drug sales, according to the Department of Homeland Security

Rivalries between the cartels wreak havoc on the lives of civilians who have nothing to do with narcotics. Bystanders, people who refused to join cartels, migrants, journalists and government officials have all been killed.

Not on news agenda

Jacob Parakilas, assistant head of the US and the Americas Programme at London-based think tank Chatham House, said part of the reason for the relative lack of attention paid to Mexico in the international media is "it's not a war in the political sense of the word. The participants largely don't have a political objective. They're not trying to create a breakaway state. It doesn't come with the same visuals. There are no air strikes.

"Also this has been going on since the beginning of the modern drug trade in the Americas. It's not news in that sense. And Mexico is one of the most dangerous countries in the world to be a journalist . They are intentionally targeted in Mexico, which puts a dampener on the ability to report on this."

Drug kingpin Joaquin "El Chapo" Guzman is facing trial in New York.

There have, however, been significant arrests in relation to the Mexican drug trade in recent times.

Damaso Lopez Nunez, a high-ranking leader of Mexico's Sinaloa drug cartel, was arrested on May 2 in Mexico City and could face charges in the US, authorities said.

His arrest follows January's extradition of Joaquin "El Chapo" Guzman , who is accused of running the Sinaloa cartel -- one of the world's largest drug-trafficking organizations.

He awaits trial in New York on 17 counts accusing him of running a criminal enterprise responsible for importing and distributing massive amounts of narcotics and conspiring to murder rivals.

World conflict deaths fall

The number of conflict fatalities globally edged down last year, from 167,000 to 157,000, according to the IISS.

This was the second successive annual drop -- 180,000 people were killed in 2014.

The number of deaths in Syria fell from 55,000 in 2015. But there were 1,000 more deaths in Afghanistan last year than 2015 and 4,000 more in Iraq.

Voronkova from the IISS said: "Civilians caught amid conflict arguably suffered more than in the preceding years. Between January and August, 900,000 people were internally displaced in Syria alone."

The internal displacement figures were 234,000 for Iraq and 260,000 for Afghanistan.