Bulgaria has expressed solidarity with Belgium which held a day of national mourning on Friday for the 22 children and six adults who died in a bus crash in a Swiss alpine tunnel as the bodies of the victims were flown home.

The remains of the 28 victims were loaded into Belgian military planes in Sion, Switzerland, a day after the parents undertook the tragic task of identifying their children and visited the site of Tuesday night's accident.

The Bulgarian Association of Victims in Car Accidents left 22 white carnation flowers at the Embassy of Belgium in Sofia as a sign of Bulgaria's solidarity with the mourning of the Belgian crash victims.

"The flowers are white because children are pure and untainted – this is the color of angels," explained Vladimir Todorov, head of the Bulgarian NGO.

"Pain is the same no matter whether kids die in Bulgaria, Afghanistan, or in the heart of Europe," he added.

The Bulgarians who came to the Belgian Embassy to express their grief for the Belgian victims were met by Belgian Ambassador to Bulgaria H.E. Marc Michelsen, who said he received calls and letters from people in Bulgaria who wished to declare their sympathies for the families of the crash victims in Switzerland.

Forty-six children and four teachers from two Belgian schools were returning home from a skiing holiday late Tuesday when their coach slammed into a concrete wall in the motorway tunnel in southern Switzerland.

Family members laid flowers at the crash site on Thursday after visiting the bodies at the morgue.

Three of the injured children remained in critical condition, a Swiss hospital spokeswoman said Thursday, and could not be moved.

The body of the driver was also expected to remain as "health analyses have to be carried out" to check if he was suffering from an illness that could have caused the accident.

After police said they did not believe the driver had been speeding, Swiss authorities said there would be a rethink about safety designs in the 2.5-kilometre (1.5-mile) tunnel.

It is believed that the coach clipped a kerb before it slammed into the wall of a rectangular emergency stop area.

A 100 kilometre (60-mile) per hour speed limit was also questioned by the press.