DEBKA from Israel was onto this about an hour ago ahead of the BBC saying there was a ‘mushroom cloud’ and the “series of large explosions” occurred at the ‘military section’ of the airport.
“Syrian ammunition depots”....”five attacks”....”manned by Iranian forces”.....”Syrian rebel sources” according to a report at 105am Eastern this morning from Arutz Sheva in Israel.
This article asserts quite forcefully that it was Israeli airstrikes on Hezbollah armaments. They don’t report any definite confirmations (Israel says “no comment”), but there is a fair amount of detail about what may have occurred:
http://www.smh.com.au/world/damascus-explosion-large-blast-fire-at-airport-20170427-gvtp9p.html