China on Thursday slammed what it called the "smearing" of its Belt and Road infrastructure project after Italy said it would withdraw four years after it became the only G7 nation to sign up.

"China firmly opposes smearing and undermining of cooperation on the joint construction of the Belt and Road," foreign ministry spokesperson Wang Wenbin said.

Beijing is also opposed to "confrontation and division among camps that causes separation", Wang added.

The foreign ministry did not directly comment on Italy's decision to withdraw from Beijing's vast BRI project, a central pillar of President Xi Jinping's bid to expand China's clout overseas.

But Wang pointed to the more than 150 participating countries as evidence of it being the "most popular international public product and the largest international cooperation platform in the world today".

He also noted that Italy had sent representatives to Beijing's BRI forum in October.

"This embodies the huge appeal and global influence of jointly constructing the Belt and Road," he said.

An Italian government source confirmed to AFP on Wednesday that Rome had pulled out of the initiative.

Proponents of the BRI praise it for bringing resources and economic growth to the Global South.

But critics have long pointed to opaque pricing for projects built by Chinese companies, with countries including Malaysia and Myanmar renegotiating deals to bring down costs.