"Absolutely horrible with Malaysia airlines probably shot down over separatist area of Eastern Ukraine," Bildt said via Twitter shortly after the news broke.
The Malaysia Airłines Boeing 777, carrying 298 passengers and crew, was heading from Amsterdam to Kuala Lumpur when it was shot down by a surface-to-air missile in an area of eastern Ukraine controlled by pro-Russian separatists.
While both sides of the Ukraine civil conflict have blamed each other for the downing, evidence is mounting to indicate that the separatists are responsible.
"Heavily incriminating intercept of Russian speaking persons talking immediately after downing. Released by Kiev," Bildt said.
"There have been signs of Russia supplying separatists also with fairly advanced surface-to-air systems recently. Horrible," he added.
The separatists have however agreed to allow international investigators access to the crash site which is located between the Luhansk and Donetsk regions which border Russia, according to a BBC report on Friday.
The airline has said that flight MH17 was carrying at least 154 Dutch nationals, 43 Malaysians, 27 Australians, 12 Indonesians, nine British, none of whom are reported to have any link to Ukraine or the civil conflict there.
"A first class international gangster crime," was how Carl Bildt summarized the incident in an interview for TV4 news late Thursday evening.