BERLIN (AP) — Climate activists blocked a runway at Hamburg airport early Thursday, causing numerous flights to be canceled on the first day of the school vacation in the north German city.

The group Last Generation said several of its members entered the grounds of Hamburg airport around 6 a.m. (0400 GMT) and glued themselves to the runway.

“It can't yet be predicted when operations can resume,” the airport operators said. “According to current information about a dozen flights have had to be canceled. Further cancelations and diversions aren't ruled out."

Members of the group also cut through a security fence at Duesseldorf airport, in the west of the country, and blocked an access route to the runway.

In a statement, the group accused the German government of lacking a strategy to tackle the climate crisis and called for immediate measures to cut emissions in the transport sector, including ending tax exemptions for airline kerosene.

Aviation is responsible for a significant share of global emissions. If the sector were a country, it would rank in the top 10 global emitters, according to the European Commission.

Last Generation is known for its disruptive protests, blocking roads and airports to demand tougher government action to reduce greenhouse gas emissions.

German prosecutors raided the homes of several of the group’s members in May on suspicion of forming or supporting a criminal organization.