MADRID (Reuters) - Party insiders of Spain’s center-right Ciudadanos have reacted with unease over its decision to deepen an alliance with far-right Vox to secure a share of power in two regional assemblies.

Ciudadanos, Spain’s third-biggest party after national elections in April and part of the liberal group in the European Parliament, agreed at the weekend to divide up senior appointments in the Madrid and Murcia parliaments with Vox and the mainstream conservative People’s Party (PP).

Their alliance will block the left from taking control in either region but it also risks further fragmenting a domestic political landscape already marked by deep divisions following national, European, regional and local elections in recent weeks.

In April, Vox became the first far-right party with a significant representation in Spain’s national parliament since the death of dictator Francisco Franco in 1975.

It had enabled the formation of a regional PP/Ciudadanos government in Andalusia by offering legislative support after an election there in December, but the weekend agreement took that three-way alliance one stage further.

“With Vox, Ciudadanos has turned itself into the key that opens the door to far-right governments,” tweeted Adriana Lastra, national parliamentary spokeswoman for the Socialist Party of acting Prime Minister Pedro Sanchez.

Former French prime minister Manuel Valls, who ran for mayor of Barcelona for Ciudadanos last month, asked in a tweet this week if his party’s agreements with Vox represented “democratic normality or the normalization of a party of the extreme right?”

“I cannot hide my deep worry,” he added.

Two Ciudadanos sources who spoke to Reuters on condition of anonymity were also critical of any agreements with Vox.

Sanchez is busy attempting to form a national government after his party won the most votes in April. Ciudadanos has said it will vote against any Sanchez government.

But outside Spain, Ciudadanos is in the same liberal camp as French President Emmanuel Macron, who has already discussed a European alliance against the far right with Sanchez.

Ciudadanos leader Albert Rivera told journalists on Tuesday that the deals reached with Vox in Madrid and Murcia only applied to positions in local assemblies, not local government.

“There is no document on government, nor political agreements... They are personal votes for the different positions in the (local) assemblies,” he said.

Last week Ciudadanos said “any agreements on government will only be reached with the People’s Party and the Socialists and will not include other parties like (far left) Podemos, Vox or nationalist parties.”

Spain’s regions have large budgets and responsibility for health care and education.