"Condolences have been forwarded to the government," in Belgium and the Swiss government "has expressed its deep sadness," said her spokesman.
"Switzerland will do everything in its power to support the injured, their families and the families of the victims," said a statement from the finance ministry, a portfolio she also holds.
Belgian Prime Minister Elio Di Rupo was also on his way to Switzerland and two army units were mobilised to transport victims' families, from two towns in Flanders.
Di Rupo flew to Switzerland as parents of the children returning from a skiiing holiday gathered at one of their schools before also heading for Switzerland aboard a government plane.
A total of 28 people died in the crash near the town of Sierre in a motorway tunnel on Tuesday night, Swiss police said, including the two drivers.
The bus had 52 people on board.
"We do not yet know the cause of the accident," said Valais canton police spokesman Renato Kalbermatten was quoted as saying on the Swiss German television channel SF, part of the national broadcaster.
The SF correspondent said the bus had been travelling for only about 20 minutes on a stretch of motorway with a 100-kilometre-per-hour speed limit making it unlikely that the driver was drowsy.
The process of identifying the bodies of the victims had begun, he added.