In the meantime, numerous countries have revived border controls 20 years after the so-called Schengen agreement permitted largely passport-free travel among European Union members.

The European Commission, the executive arm of the European Union, warned on Tuesday that it might soon authorize member states to extend border controls for up to two years instead of the normal six months, if Greece does not quickly outline how it plans to tighten control of its borders.

The commission said Greece had still not made adequate progress in registering thousands of migrants throughout the country. Analysts and European politicians have warned that reinstating border controls could deal a sharp blow to the European Union economy.

Greece, which does not have the resources to process and assist the migrants, has been waiting for more than 1,000 police officers and asylum experts to arrive from other European countries to help.