A jet carrying Macedonian Prime Minister Nikola Gruevski made an emergency landing in Zurich on Thursday due to technical problems but no one was hurt in the incident, a spokesman said.

"The (technical) failure manifested as a strong noise, from an unknown origin, and it was followed by perturbation of the cabin pressure," a government spokesman told reporters in Skopje.

A 25-year-old Swiss man suspected of trying to travel to Syria or Iraq to join jihadists has been arrested at Zurich airport, the federal prosecutor's office said on Wednesday.

The man, from the Zurich area, was arrested on Tuesday while seeking to board a plane to Istanbul, Turkey.

Two trains slammed into each other near the Swiss city of Zurich early Friday, tipping over carriages and injuring at least six people, police said.

The driver of one of the trains was in serious condition after the crash, which saw ambulances, fire and rescue services rush to the scene as a rescue helicopter hovered above.

A bomb threat caused flight delays and the partial shut-down of Switzerland's main airport in Zurich on Monday before security forces gave the all-clear, reports and officials said.

Around a dozen flights were delayed by up to 30 minutes, police said, as a result of what they said was an unspecified threat. No flights were cancelled.

Police fired tear gas and water cannon in central Zurich late Friday to disperse some 200 masked radicals who attacked officers and set cars, trees and trash bins on fire, sending terrified residents fleeing in the upscale and normally tranquil neighborhood.

The rioters, many of them armed with iron bars, threw rocks and Molotov cocktails at police in a rampage that left hundreds of thousands of euros worth of damage and that police said was claimed by left-wing radicals.

Surgeons in Zurich have carried out a complex, multi-organ transplant, hospital officials said Thursday, claiming the lessons learned would benefit patients with tumors that attack multiple organs.

Two teams of surgeons at the Zurich University Hospital had carried out a live liver transplant and simultaneously transplanted large blood vessels and replaced part of the same patient's heart with regenerated tissue, the hospital said in a statement, describing the operation as a "world first".

Switzerland's largest city Zurich acknowledged Monday it had lost trace of 5,176 works of art, including an original painting by Swiss-born architect Le Corbusier.

The city had carried out its first full inventory of its vast collection of 35,000 pieces in nearly a century, only to discover that nearly 15 percent were missing, including nearly 1,400 original works, it said in a statement.

Serbia's interior minister said Wednesday that two Pablo Picasso paintings owned by a German museum and stolen from a Swiss exhibition have been found in Belgrade.

Ivica Dacic provided no information on the exact location of the paintings or possible suspects in the theft.