World has 'lost its ability to stop conflict' and is failing to answer challenges of climate change, population and food security

The UN high commissioner for refugees has warned that the world is standing on the precipice of a global humanitarian crisis, one that the international community is unprepared and ill-equipped to face.

Speaking on Monday at the annual forum of InterAction, an alliance of NGOs based in Washington DC, António Guterres painted a bleak picture of the decades ahead.

"As humanitarian actors we are going to find the next two to three decades ones of unparalleled challenges and we are unprepared for what is to come," he said.

"We are facing increasing humanitarian challenges because … the international community has lost its ability to stop conflict … power relations have become unclear and unpredictability has become the name of the game.

"We are not coming up with any collective answers to emerging global mega-trends, such as climate change, population growth, urbanisation and food security … with dramatic human consequences."

This, he told an audience of more than 1,000 NGO workers, policymakers and political analysts, will inevitably have serious repercussions for humanitarian aid operations. "Humanitarian aid budgets are not growing proportionally to humanitarian needs," Guterres said. "We are seeing how difficult it is to mobilise resources to address the dramatic impact of the Syrian crisis. We will be called to do more and more with less."

Guterres pointed to increasing problems with access to crisis areas, and the issue of national sovereignty being used to deny people their rights.

"The very nature of conflict is changing," he said, adding that the humanitarian world has not caught up with the reality of how modern conflicts are fought. "We no longer have wars where country [fights] against country or governments against a clear opposition, which makes our ability to assess the population we need to [help] difficult."

The failure of the UN security council to halt or defuse the war in Syria has proved catastrophic, he said. The conflict is now the biggest threat to global security as well as a humanitarian black hole of unprecedented suffering and scale, he added.

The high commissioner, who has led the UNHCR through the worst of the refugee crises in Afghanistan and Iraq, this month called the Syrian conflict the worst humanitarian crisis since the end of the cold war and the most serious the UN body has had to contend with.

About 8,000 people are fleeing the country each day. The UN's most recent data estimates that 6.8 million Syrians need aid. This figure is likely to reach 10 million, half of Syria's pre-war population.

The deliberate targeting of medical staff during the Syrian conflict is particularly concerning for the humanitarian community. Hundreds of local doctors and medical staff have been killed since violence erupted in March 2011, with thousands more fleeing the country, according to the UN's independent international commission of inquiry on Syria.

"I recognise that the [humanitarian sector] is doing better but these gains are out of proportion to the needs we face to adapt to a changing world more quickly," Guterres said. "We still do business in a very traditional way. We need to take more advantage of technology and the benefits this can bring to the people we care for. We are still very scattered in terms of doing advocacy."

He pointed to the failure of many UN agencies to respond effectively to fast-moving and complex humanitarian situations unfolding in conflict zones. He criticised a tendency to try to integrate political security and humanitarian work within the UN system.

Speaking after the event, George Rupp, chief executive of the International Rescue Committee, said targeting health professionals "signals a new and destructive shift and one that fundamentally threatens the humanitarian space in which we all operate". That space, said Rupp, is being squeezed as aid workers find their access to conflict zones restricted and their impartiality threatened.

"We are seeing the nature of conflict become more brutal and more ruthless in its approach to local civilian populations. The ability of humanitarian actors to respond to these new threats must be protected and we must all work together to find ways of ensuring our work continues," he told the Guardian.

Guterres's speech coincided with the release of a report by the Internal Displacement Monitoring Centre in Geneva that said the number of people internally displaced by armed conflict, violence and rights violations had reached record-high levels of almost 29 million.