SAN SALVADOR/BARCELONA: A common sight on any street corner in the poor neighbourhoods of El Salvador's capital is a teenage gang member controlling territory by standing guard with a mobile phone in hand, checking people and cars that enter and leave.

Gang violence has plagued El Salvador for decades with entire city districts controlled by the two most powerful gangs - Barrio 18 and its rival Mara Salvatrucha - who use sexual violence, extortion and death threats to cement their grip over communities.

For aid agencies working in slum neighbourhoods in parts of Latin America, gang warfare, including shoot-outs, and the violence it brings have become the main challenge.

The Internal Displacement Monitoring Centre said this month that criminal violence associated with drug trafficking and gang activity had displaced at least 1 million people in El Salvador, Guatemala, Honduras and Mexico as of December 2015.

"We are living a humanitarian crisis that it is a product of the violence. Gangs create terror and live in communities paralysed with fear," Rodrigo Bustos, El Salvador director for child rights group Plan International, told the Thomson Reuters Foundation. "Our programmes can't be business as usual."

A key issue at the first World Humanitarian Summit next week will be the need for aid groups to adapt to fast-changing realities in mushrooming cities in the developing world.

To find better ways of responding to floods, earthquakes, armed violence, migration and other problems in cities, a Global Alliance for Urban Crises will be launched at the summit, bringing together UN agencies, international aid groups, the private sector and researchers.

In El Salvador, for example, some agencies have been forced to scale back their work in gang-controlled areas, while others, like the US Peace Corps, have suspended their projects entirely.

"We offer medical services to everyone and we know that we can be assisting a wife or girlfriend of a mara (gang member)," said Henry Rodriguez, who runs the Honduras and Mexico mission for aid group Medecins Sans Frontieres.

"When the atmosphere is tense in a neighbourhood, we back off and stay away for several days and come in again when things have calmed down."

Gaining acceptance and trust among local people, which involves speaking to religious and community leaders, is vital but can take years. Most aid agencies also have to negotiate with gangs to get permission to enter their areas and work.

"NOT CRIME-PROOF"

How to provide healthcare and other essential services to poor slum communities in such risky urban conditions is a challenge many humanitarians - who have traditionally operated in rural areas - are still ill-equipped to tackle, experts say.

But the urgency is growing as the problem has spread beyond Latin America, affecting cities in Central African Republic to Jamaica and Iraq, said John de Boer, a senior policy advisor with the United Nations University's Centre for Policy Research.

"There are a number of places where these kinds of alternative forms of criminal governance have established roots, creating tremendous humanitarian issues - in some cases profiting from them," he said.

Organised crime gangs can also have hidden effects on aid work, for example by providing construction materials for anti-flood levees or pushing people to back certain projects.

"A lot of our work is not crime-proof," de Boer said. "If you don't go into those contexts with a very good sense of the lay of the land when it comes to the impact of illicit groups... you could be giving rise to huge problems down the road."

Cities caught in a toxic mix of violence, inequality, natural disasters and poverty are the most fragile, he said, and many lack municipal governments capable of resolving these problems.

With low- and middle-income countries estimated to host 95 percent of future urban expansion, much of it rapid and unplanned, aid agencies will increasingly be required to respond to crises in cities, said Pamela Sitko, a technical advisor on managing urban disasters with World Vision International.

"It's too late for the humanitarian sector to get ahead of the urbanisation curve, but we can certainly catch up. We need to adapt our tools, approaches, ways of partnering," she said.

LOCAL GOVERNMENTS IGNORED

One of the main differences is that aid workers will have to cooperate far more closely with the people who run cities - from mayors and urban planners, to architects and companies supplying water and electricity.

In disaster-hit urban areas, the best approach is often not to bring in emergency relief supplies. "In cities, we don't provide aid to people - we need to facilitate access to aid. For example, it doesn't mean trucking in food, it means helping markets get back and working," Sitko said.

Dan Lewis, head of UN-Habitat's urban risk reduction unit, said the humanitarian system is gradually learning from mistakes made in previous disasters that have struck cities.

They include the 2010 Haiti earthquake, when thousands of homeless residents from Port-au-Prince were moved to a camp outside the capital with no access to jobs and few services.

In many crises - from the Indian Ocean tsunami to Typhoon Haiyan which flattened the Philippine city of Tacloban - aid agencies have rushed in without consulting local governments.

The new global alliance aims to rectify this oversight.

Aid experts say cooperating with municipal authorities and businesses is becoming ever-more important in dealing with refugee crises like that in the Middle East where most of the nearly 5 million Syrians who have fled their war-torn country are living in towns and cities in Lebanon, Jordan and Turkey.

Their dispersal among host communities has made it harder for aid agencies to reach families with the support they need to pay rent, buy food, and access healthcare and schools.

This requires innovation. World Vision, for example, runs a project in Lebanon's Bekaa Valley that gives Syrian and Lebanese youth experience in managing solid waste and recycling materials. It helps them bond and acquire skills, while improving local public health and municipal infrastructure.

"You need to serve all the different groups," said Aline Rahbany, World Vision urban programming adviser. "If you only help one, it could lead to harm by causing more social tensions and conflict in a city."