Former Balkan rivals pledged joint efforts with the United States and the European Union in protecting European borders from a record surge of asylum - seekers amid fears that Islamic militants may have infiltrated the flow. Southeast European leaders met at an urgent summit in the Croatian capital attended by U.S. Vice President Joe Biden to discuss the crisis that has seen hundreds of thousands of people fleeing war and poverty in the Middle East, Asia and Africa cross the Balkans. "The present dramatic migrant crisis poses and unprecedented challenge from both humanitarian and security aspects, requiring dialogue and agreement to better protect the EU's external borders and substantially alleviate migrant pressure on affected countries," a declaration issued after the summit said. Without outlining concrete border measures, the declaration added that for lasting peace and stability of the once war-ravaged region, all countries should have the option of joining the European Union and NATO. The summit included all former Yugoslav states and Albania, of which only Slovenia and Croatia are both in the European Union and in NATO. Biden's trip came amid a heated debate in the U.S. about whether to admit Syrian refugees following the Islamic State group's Nov. 13 attacks in Paris that left 130 dead and hundreds wounded. At least two of the militants involved in the Paris attacks had passed through Greece, apparently posing as asylum-seekers. In Washington, the House of Representatives has passed laws tightening vetting for refugees from Syria and Iraq, although the White House contends the additional restrictions will effectively block people from those countries. "The refugee crisis is restraining the resources of the countries in the region," Biden said. "It is clear there is a need to improve cross-border cooperation, information sharing to deal with the flow of refugees, while stepping up the capability to counter the terrorist threats as well." Analysts believe Biden's appearance indicated renewed U.S. interest in the region, which was engulfed in a bloody civil war in the 1990s as the former Yugoslavia broke up. But Biden said: "Our presence is not a return, we never left." "The United States of America, and me in particular speaking for the president of the United States, has had an overwhelming interest in this region for the last 25 years," Biden said. The refugee crisis is stoking tensions among the countries on the migrant corridor — Greece, Macedonia, Serbia, Croatia and Slovenia. Managing the massive refugee flow has proved a major political and security challenge — and Europe's inability to agree on a common policy toward the surge has made the situation worse. Slovenia wants to limit the flow of people talking the Balkans route — from Greece through the Balkans up to Germany — so it can re-establish the normal functioning of Europe's passport-free Schengen border zone. Slovenia has started to erect a razor wire fence along its border with Croatia to aid these efforts but Slovenian President Borut Pahor said getting an agreement with its southern neighbor should be its main priority. The Balkan nations now only let in people fleeing conflicts, such those from Syria, Iraq and Afghanistan, and not people considered economic migrants.