Hungary's prime minister has said eastern European nations are forced to protect themselves from the "brutal threat" of mass migration until the EU organises a united response to the refugee crisis.
Prime Minister Viktor Orban's government has built a fence to keep out refugees on the Serbian border and is rapidly constructing a similar 3.5-metre-high steel fence on the Croatian border.
The Hungarian government has been warned they "will have to open the border" because of the buildup of refugees on the Serbian side.
Aleksandar Vulin, Serbia's minister in charge of tackling the migrant crisis, said discussions were ongoing with Hungarian officials but the crossing would have to be re-opened.
Elsewhere, the UN High Commissioner for Refugees (UNHCR) said refugees would be likely to find new routes into the EU if the Hungarian border remained closed.
"We're definitely in touch with different countries on contingencies and UNHCR is ready to move and assist different countries as best we can," UNHCR spokeswoman Melissa Fleming said.
International development secretary announces extra resources to help identify and prioritise the most vulnerable refugees from camps.
Thousands of refugees and migrants have been trapped in a cruel tug-of-war overnight as European nations squabble over what to do with them.
Croatians have been warning refugees and migrants who are attempting to cross the country's border to be aware of minefields in the area.