Spain's central government is threatening to deploy national police to ensure security in Catalonia if regional authorities fail to stop the recent disruptions by pro-independence protesters on major highways.

The warnings, in writing on Monday and also in public remarks on Tuesday by officials in the center-left Cabinet of Spanish Prime Minister Pedro Sanchez, follow the blocking of a highway that runs across Catalonia for more than 15 hours on Saturday.

Separatist groups who want Catalonia to become a republic after a banned referendum was held there last year, returned to protest the next day, on Sunday, allowing cars to pass through tollbooths without paying the mandatory fees.

The presence of Spain's national police or civil guard officers in Catalonia is a highly contentious issue, even more so because of their role in the violent crackdown on the Oct. 1, 2017 secession vote. Separatists often called them "invaders."

Last year's independence attempt in the northeastern region led to an unprecedented political crisis, but Catalan and Spanish authorities began talks in June when Sanchez's new center-left administration took power.

The political thawing appeared to wear off Tuesday amid hardening rhetoric on both sides. Spanish officials have criticized Catalan President Quim Torra, a separatist, for saying that Slovenia's secession from Yugoslavia in 1991 was the roadmap for the Catalan independence movement.

Although many European and other countries were quick in recognizing Slovenia's independence, The Ten-Day War that followed its declaration of independence caused dozens of casualties on both sides and hundreds were injured, a fact Spanish authorities highlighted in their criticism of Torra.

Although the Catalan official hasn't commented on the controversy, a regional government spokeswoman said Tuesday that Catalonia should follow its own "peaceful path" to independence while looking for "inspiration" in other countries' experience with secession.

In the latest bout of tension, three Cabinet members of Spain's government sent letters late Monday urging their Catalan counterparts to explain why Catalan police force Mossos d'Esquadra, which is controlled by the regional government, didn't intervene to stop protesters from blocking highways over the weekend.

"Apparently, there has been a dereliction of duties, with economic, social and public security consequences," wrote Deputy Prime Minister Carmen Calvo in her letter to Catalan Vice President Pere Aragones.

The prosecutor's office in Catalonia also opened an investigation Tuesday into the highway disruption, mentioning that regional police officers at the scene "didn't use the necessary and proportional force to re-establish public order."

Spanish Interior Minister Fernando Grande-Marlaska said in a separate letter to his Catalan counterpart that if the Mossos don't end public disorder and traffic disruptions, "the government will order an intervention of the state's security forces."

By sending the warning, Sanchez's government seemed to be preparing the ground for an anti-riot police deployment before a weekly meeting of Spain's government that the prime minister wants to hold in the Catalan regional capital, Barcelona, on Dec. 21. A pro-independence Catalan union has called a region-wide strike to mark the day and protest the presence of the Spanish Cabinet in full.

The Catalan government on Tuesday said it would respond to Madrid's letters. The regional spokeswoman, Elsa Artadi, also called on central authorities to "walk on the path of dialogue, negotiation and political solutions."