GENEVA (Reuters) - A rapid build-up of migrants at Greece’s northern borders risks creating a humanitarian disaster, the United Nations High Commissioner for Refugees (UNHCR) said on Tuesday.

The agency called for better planning and accommodation for at least 24,000 stuck in Greece, including 8,500 at Idomeni where hundreds of migrants stormed the border on Monday and Macedonian police fired teargas to disperse them.

“Europe is on the cusp of a largely self-induced humanitarian crisis”, the UN refugee agency’s spokesman Adrian Edwards told a news briefing.

“The crowded conditions are leading to shortages of food, shelter, water and sanitation. As we all saw yesterday, tensions have been building, fuelling violence and playing into the hands of people smugglers,” he said.

Migrants have become stranded in Greece country after Austria and countries along the Balkans migration route imposed restrictions on their borders, limiting the numbers able to cross.

The UNHCR also urged all EU member states to reinforce their capacity to register and process asylum seekers through their national procedures as well as through the European relocation scheme.

“Greece cannot manage this situation alone,” Edwards said.

Despite commitments to relocate 66,400 refugees from Greece, states have so far only pledged 1,539 spaces and only 325 people actually have been relocated, he added.

Some 131,724 refugees have crossed the Mediterranean in January and February, slightly less than in the first half of 2015, it said. A further 410 have died at sea.

(In final paragraph, UNHCR corrects refugee figure for first half of 2015, which is less than, not more than, the number so far this year)