Rex Tillerson spoke as he welcomed Belgium's Foreign Minister Didier Reynders to the State Department, and officials confirmed that a Belgian woman was among those killed in the assault.
Granting Catalan leader Carles Puigdemont political asylum in Belgium would be "not unrealistic" if he asks for it, the Belgian migration minister said, underlining his country's position as a contrarian voice in the Spanish standoff.
Belgium is setting up a new English-language court to settle international business disputes in the latest move by an EU country to try to lure companies away from London after Brexit.
Security checks for passengers boarding high-speed trains have been increased at three major Belgian stations since last month.
Belgian biscuit maker Lotus Bakeries said it had recalled some of its products in Belgium as they could contain eggs that were contaminated with insecticide fipronil.
Belgium has ordered Paris terror suspect Salah Abdeslam and another man to stand trial over a police shootout with Belgian police last year, prosecutors said on Thursday.
The European Court of Human Rights on Tuesday upheld a Belgian ban on wearing the full-face niqab veil in public.