Huge traffic queues have formed on a Hungarian motorway leading to Austria after border controls were tightened in a bid to clamp down on human trafficking.
Austrian officials put tougher checks in place on the country's eastern border after the bodies of 71 migrants were found in the back of a lorry abandoned on a motorway.
Five people have been arrested in connection with the incident.
Governments must do "much more" to solve the migrant crisis, the UN's Secretary-General has said.
Ban Ki-moon said he was "horrified and heartbroken" by the recent deaths, including the "grim" discovery of 71 bodies in a lorry found abandoned on an Austrian motorway.
He said: "I commend those leaders and communities who have stepped up to our shared responsibilities and obligations. But much more is required.
"I appeal to all governments involved to provide comprehensive responses, expand safe and legal channels of migration and act with humanity, compassion and in accordance with their international obligations.
"Let us do all we can to provide people in the most desperate circumstances with a measure of safety and a sense of hope."
Earlier, Hungarians held a vigil for the victims of the Austrian discovery outside Budapest's main train station, where thousands of migrants have been sheltering for weeks.
The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.