Charles Michel, the Prime Minister of Belgium whose capital city was targeted by bombers in March, tweeted from the EU summit in Brussels: "Our thoughts are with the victims of the attacks at Istanbul's airport.
We condemn these atrocious acts of violence." In December, a blast on the tarmac at a different Istanbul airport, Sabiha Gokcen, killed a cleaner.
That attack was claimed by a Kurdish group, the Kurdistan Freedom Falcons (TAK).
A US state department travel warning for Turkey, originally published in March and updated on Monday, urges US citizens to "exercise heightened vigilance and caution when visiting public access areas, especially those heavily frequented by tourists."