The multi-billion-dollar initiative, unveiled by Xi in 2013, aims to link the continents through a network of ports, railways, roads and industrial parks.
Beijing plans to develop the network through 65 countries representing an estimated 60 percent of the world's population and a third of its economic output.
So far, around 70 countries have signed a memorandum of understanding pledging their interest in the project — an agreement that Beijing values as it seeks to expand its project.
In Europe, countries such as Poland and Greece have signed but the project has created considerable anxiety that it masks an attempted Beijing influence grab.
“We're not going to sign the initiative,” said an official from the Spanish prime minister's office, who refused to be named.
“Europe has a connectivity initiative with Asia, so we think Europeans should work within this framework,” the official said, citing his country's reason for not signing China's Silk Road plan.
He was referring to the European Union in September presenting its own plans to develop a parallel network of infrastructure to connect it to Asia.
But the Madrid statement ahead of Xi's state visit to Spain later Tuesday could irk China.
Jean-Francois Di Meglio, president of France's Asia Centre research centre, said the memoranda of understanding that countries have signed with China have little legal value.
But they are a “signal that 'I want to do business with you',” he told AFP.
“In China's mind, it's really important to get it,” he added, as Beijing pushes to revive Silk Road trade routes with its costly initiative.