The European Union is preparing to spend hundreds of millions of euros on humanitarian aid, as Greece struggles to cope with an influx of migrants.

Under plans to be submitted on Wednesday, EU funds could be deployed within Europe in the same way they are used to help crises outside the bloc.

The UN has warned of a humanitarian disaster caused by a build-up of migrants on Greece's borders.

Thousands of asylum seekers remain trapped on the border with Macedonia.

The plan to be submitted by the EU's executive body, the European Commission, means EU aid agencies would for the first time work directly with the UN and other groups inside Europe, rather than dispersing money to individual member states.

EU officials said the aid plan would allocate 300m euros (£233m; $325m) this year to help any EU state deal with the migration crisis. In all, 700m euros would be made available over three years.

However, BBC Europe correspondent Chris Morris says that even if such funding can be deployed quickly, the EU also needs to stem the flow of new arrivals. That would mean better co-operation with Turkey, he adds.

Greece has asked the European Commission for nearly 500m euros in assistance to help care for 100,000 asylum seekers.

About 24,000 migrants in Greece are in need of housing and more than 8,000 are stuck in worsening conditions on the border with Macedonia.

The crossing at Idomeni has closed following protests by migrants desperate to continue their journey to countries further north, particularly Germany.

Despite the border closure, thousands more migrants and refugees are still travelling to Idomeni. With the camp overwhelmed, many have been forced to wait on buses and at petrol stations along the route from Athens.

Some migrants have been stranded at the overburdened camp for more than a week.

Migrants and refugees rest in a camp as they wait for a train to Serbia near Gevgelija on the Greek - Macedonian border on February 25, 2016. Photo: Robert ATANASOVSKI / AFP

Caroline Haga, an emergency co-ordinator for the International Red Cross (ICRC), said the Greek military had set up three more border camps near Idomeni but all are already full, with about 2,000 people at each.

On Tuesday, the UN refugee agency (UNHCR) warned that Europe was "on the cusp of a largely self-induced humanitarian crisis".

Spokesman Adrian Edwards said crowded conditions on Greece's borders were leading to shortages of food, shelter, water and sanitation.

Last week Macedonia began refusing entry to Afghan migrants and imposing stricter document controls on Syrians and Iraqis, slowing the passage to a trickle.

Then on Monday, Macedonia firmly closed the gate that separates it from the camp, citing the closure by Serbia of its northern border.

A similar knock-on effect has happened previously as Eastern European countries attempt to reduce the number of migrants crossing their borders.

Frustration at the closure boiled over on Monday and scores of migrants clashed with security guards. Some attempted to smash through a fence, prompting the guards to fire tear gas and rubber bullets.

European Council President Donald Tusk is due to visit Croatia and Macedonia on Wednesday before moving on for talks in both Greece and Turkey in advance of a special EU summit next Monday.