Effective at 4pm local time today (17 April 2010) the Airport Nikola Tesla in Belgrade (Aerodrom Nikola Tesla/Аеродром Никола Тесла) is closed because of the ash cloud from the volcano in Iceland.
[Update 04-18-2010 06:39:59] As of now, the Belgrade Airport is still closed. For more information, read this article on the expanding ash cloud.
[Update 04-19-2010 07:58:28] The airport website has a notice stating that it reopened to traffic at midnight last night through midnight tonight.