Serbian authorities said two migrants died and two others were in serious condition after hiding in a tanker truck near the border with Hungary.
The men were discovered on May 21 in northern Serbia when the truck driver opened the tank and saw them lying unconscious.
Hospital officials in Novi Sad told RFE/RL’s Balkan Service that the migrants suffered heatstroke and suffocation. One died on May 22 and the other died overnight. The two others are in intensive care.
According to the Serbian Interior Ministry, at least two of the migrants were from Afghanistan.
Thousands of migrants fleeing war and poverty have been moving through the Balkans in a bid to reach wealthy nations in the West.
But up to 4,000 remain stranded in Serbia after the EU closed the so-called Balkan route in 2016.