Spanish Prime Minister Mariano Rajoy rejected a request from the leader of Catalonia, Saturday, to approve a referendum allowing the northeastern region to vote on seceding from Spain. The rebuff comes three days after an estimated 500,000 Catalan demonstrators linked arms to form a 250-mile human chain to push for independence.

"The ties that bind us together cannot be undone without enormous cost," Rajoy wrote to Artur Mas, head of Catalonia’s regional government. The letter officially denied the pro-separatist request Mas made in July.

Spain's constitution says only the central government can call a referendum.

Growing calls for secession from Spain in the region of 7.5 million people has added to pressure on Rajoy, who is already mired in a corruption scandal and aiming to pull his country out of an economic recession.

"I am convinced of the exceptional relevance of Catalonia for Spain and of the richness, plurality and singularity of the Catalan society," Rajoy said in the letter.

"And, of course, I also want to pass on the strong belief of my government that we need to work together to strengthen these ties and move away from confrontation," he added.

In the letter, Rajoy also proposed to hold talks with Mas.

Rajoy and Mas met in secret in August to end a year-long standoff over the referendum and greater tax powers for the region, but they have made little progress and largely disagree on the nature and the scope of the talks.

Rajoy's letter did not specify which issues should be part of the proposed talks. He has said in the past that a vote on Catalan independence would be unconstitutional and has pledged to block it in the courts.

Although he made no direct reference to a possible referendum in the letter, his office said his position remained unchanged.

Mas said earlier this week that he would be ready to negotiate a new tax regime more favorable to the region, but insisted it would not be enough and that a vote should also be held.

He has threatened to call for an early election and use it as a plebiscite on secession if Rajoy uses the courts to block a referendum. Polls show backing for secession has risen steadily in Catalonia, with some recording support as high as 50 percent.

Catalonia is an economically powerful region of Spain, comprised of four provinces including Barcelona, Spain's second-largest city, and is one of the country's most industrialized and populous areas.

The region has been a part of Spain since the 15th century, when King Ferdinand of Aragon and Queen Isabella of Castile married and united their kingdoms to form one nation. Catalonia has had varying degrees of autonomy since it came under Spanish rule, and it currently operates as a semi-autonomous region within Spain.

Al Jazeera and wire services