EU chief Ursula von der Leyen dispatched a senior official to Greece on Wednesday and promised Brussels will offer support to refugees after a fire ravaged a major migrant camp.

"I am deeply sorrowed by last night's events at the Moria refugees camp in Greece," the president of the European Commission tweeted, adding that she had sent on of her vice-presidents, Margaritis Schinas, on a urgent trip back to his homeland. 

"We stand ready to support, with member states. Our priority is the safety of those left without shelter."

Thousands of asylum seekers have been left homeless by the fire in the overcrowded Moria camp on the Greek island of Lesbos, already a potent symbol of Europe's struggle to agree a refugee policy.

Schinas is the EU commissioner for migration, security and "promoting the European way of Life."

Germany has called on EU member states to pitch in to help resettle the refugees, but so far Brussels has announced only that it will help 400 children and teenagers among the 12,000 residents.

"The safety and shelter of all people in Moria is the priority," Internal Affairs Commissioner Ylva Johansson tweeted after talking with Greek authorities. 

A major rescue operation was under way Wednesday in the largest refugee camp in Greece. 

"I have already agreed to finance the immediate transfer and accommodation on the mainland of the remaining 400 unaccompanied children and teenagers," Johansson added.

The crisis in Moria illustrates the need to reform the EU's migration policy, a process long stalled by divisions between member state governments.

The European Commission is due to present a proposal, which has been postponed several times, for a "new Pact on Migration and Asylum" at the end of the month. 

The EU executive is also coordinating a program of relocation to ten EU countries of some 2,000 unaccompanied minors in Greek refugee camps. 

So far about 640 children, adolescents and families of sick children have been relocated to Belgium, France, Luxembourg, Germany, Ireland, Portugal or Finland.