The prime ministers of four central European countries say they want to preserve the European visa-free Schengen zone that is threatened by the current wave of migrants. The United Nations refugee agency says it's "deeply concerned" about the violence at the border between Greece and Macedonia where scuffles have broken out between different migrant groups. Greek authorities say they have recovered the body of a woman and are still searching for three other missing migrants after their small boat sank in the eastern Aegean Sea. The boat was carrying a total of seven migrants, and the coast guard says three survivors were pulled from the sea after Thursday's capsizing off the island of Farmakonissi. Greece is at the forefront of Europe's immigration crisis, with more than 700,000 people having crossed over so far this year from nearby Turkey, in rickety boats provided by smuggling gangs charging high fees. Greece is at the forefront of Europe's immigration crisis, with more than 700,000 people having crossed over so far this year from nearby Turkey, in frail boats provided by smuggling gangs charging high fees. Pressure is mounting on Greece to ensure better control of its borders and register arriving migrants or face the prospect that passport checks could be reintroduced for Greek citizens in Europe. The European Union urged Greece on Thursday to step up controls on its sea and land borders, and EU interior ministers will discuss on Friday the state of border controls in the country. British authorities say a Palestinian man suspected to be the leader of an organized criminal gang that smuggled thousands of migrants into Europe faces extradition to Greece. Authorities say the gang, based in Greece, is believed to have earned nearly 10 million euros ($ 10.6 million) since 2013 by charging migrants for transportation, forged documents and housing. Serbia's prime minister has urged a joint international effort to solve the migrant crisis, warning that xenophobia and fear of extremists have been on the rise since the Paris attacks that killed 130 people. Hungary's prime minister says the country is filing a lawsuit against the European Union objecting to a mandatory plan to distribute migrants among members of the bloc. Parliament last month approved legislation obliging the government to challenge the EU quotas, while Orban has repeatedly said that Brussels overstepped its authority when it approved the scheme despite opposition from a handful of mainly Eastern European countries. Greek riot police have been deployed in the area, forming a barrier to protect refugees waiting in a field on the Greek side to cross the border. Other migrants lowered the man's severely burned body to the ground and covered it with a sheet, and a volunteer doctor in the area confirmed the man had died. Groups of migrants on Thursday used empty barrels, pieces of wood and metal to make a barrier about 120 meters (feet) from the Greek-Macedonian border and are stopping all Syrians, Afghans and Iraqis from entering the area.