Swiss airspace reopened Wednesday morning after a computer glitch grounded flights across the country for several hours, officials said.

"Swiss airspace is now open again," the Alpine nation's air traffic control service Skyguide said on Twitter, adding "the technical malfunction at Skyguide has been resolved".

It did not say what had caused the problem that shut Swiss airspace for hours Wednesday morning, but said that "air traffic over Switzerland and operations at the national airports of Geneva and Zurich are resuming".

Those airports too announced that flights had begun taking off.

"Good news! Air traffic has gradually resumed since 8:30 am (0630 GMT)," Geneva airport said in a tweet, warning that a number of flights had been cancelled and urging passengers to check with their airlines.

Zurich airport also said flight operations were "running again" at full capacity by 10:00 am (0800 GMT).

But it warned that "delays are to be expected today. We still recommend passengers to check with their airline about the status of their flight."

- 'Vacation could fall through' -

At Geneva airport, where the first morning flights were delayed by more than three hours, dozens of travellers crowded around the information screens, with phones stuck to their ears.

Airport spokesman Ignace Jeannerat told AFP that around 2,000 people had seen their flights affected, adding that while flights were resuming, there would not be a return to normal before early Thursday.

"We are trying to find a solution," Sandrine Vert, 52, told AFP, after her family's easyJet flight to Split in Croatia was cancelled.

She, her husband and teenage daughter, who drove to the airport from Annecy in France, had been told there were now no flights from Geneva to Split until Friday, putting their one-week holiday plans on the line.

"Our vacation could fall through," she said.

The chaos erupted when Skyguide announced it had "experienced a technical malfunction in the early hours of this morning, which is why Swiss airspace has been closed to traffic for safety reasons".

It said it regretted "this incident and its consequences for its customers, partners and passengers."

The closure blocked all overflight of the country, and forced a number of flights to be rerouted to airports in neighbouring countries.

The Swiss news agency ATS said some international flights to Switzerland had been re-routed to Milan in northern Italy.

Zurich airport's website meanwhile showed that a United Airlines flight from New York had been rerouted to Frankfurt in western Germany, while a Singapore Airlines flight from the city state had been sent to the southern German city of Munich. 

Zurich is Switzerland's largest airport, with more than 10.2 million passengers going through its terminals in 2021.

But with Covid restrictions lifted, air traffic has picked up significantly since then, with 1.9 million passengers registered there in May alone. For this summer season, it is serving flights to 191 destinations.

Geneva airport saw 5.9 million passengers pass through during the Covid pandemic year of 2021. There too, traffic has been rapidly picking up, and it now sees around one million passengers a month.