MADRID (AP) — Spain’s government said Thursday it is mobilizing more than 15,000 police to crack down on striking truck drivers blamed for attacks on colleagues who haven’t joined the open-ended walkout.

Transport Minister Raquel Sánchez claimed “a group of extremists bent on blackmailing this country” is behind the attacks, saying they were linked to far-right groups. She offered no evidence for her claim.

The attacks by roadside picketers this week have allegedly included throwing rocks at trucks on the road, tearing cargo tarps and puncturing truck tires, as well as threatening working drivers with violence.

High fuel prices prompted the walkout.

Police across Spain have orders to ensure that essential goods are delivered, that essential services are maintained, and that those truckers who want to work can, the Interior Ministry said.

The open-ended strike, which began Monday, is not supported by Spain’s main transport trade unions nor its road haulage federations. The government says the strikers are a minority of the country’s truckers.

Even so, their walkout threatened to disrupt national supply chains, with some businesses reporting shortages of fresh produce such as milk and fish.