Frankfurt airport, Germany's busiest, has resumed operations after suspending flights on Thursday morning when several climate activists blocked runways by gluing themselves to the tarmac.

Planes were able to take off and land again, the airport said on its website, but passengers were still advised to check on their flight status before setting off for the airport.

A spokesperson for Frankfurt, a vital international transit hub and one of Europe's biggest airports, said around 140 flights had been cancelled out of 1,400 planned on Thursday.

Last Generation climate activists said in a statement that six protesters had cut through a fence and had reached various points around the Frankfurt airport runways with posters reading "Oil kills". Images released by the group showed protesters in orange safety vests with their hand glued to the tarmac.

A federal police spokesperson said several climate activists were on the airport grounds.

The group, which wants the German government to pursue a global agreement to exit oil, gas and coal by 2030, has listed several countries across Europe and North America where similar disruptions are planned as part of a protest campaign that began on Wednesday.

In Norway, around a dozen activists blocked a part of the check-in area of the Oslo Airport, in a second day of actions, but there was no disruption of flights to and from the Norwegian capital, an airport official said.

Germany's Cologne-Bonn airport, the country's sixth-largest, suspended flights for several hours on Wednesday after climate activists glued themselves to a runway, while similar actions at other European airports had been foiled by authorities.

German Interior Minister Nancy Faeser, whose ministry wants to punish unauthorized airfield access with up to two years in jail, called the action "dangerous, dumb and criminal."