Russian company Stroytransgaz, which had been commissioned the construction of South Stream in Bulgaria, is pulling out of the gas pipeline project.

The announcement was made by the subcontractor's majority shareholder Gennady Timchenko in an interview with Russian news agency ITAR-TASS.

Gazprom's subsidiary Centrgaz is now due to replace the company as the main South Stream builder in Bulgaria.

"We have left the project in order not to expose it to a risk of collapse," Timchenko told ITAR-TASS.

He alleged that US Senator John McCain had "brought local [Bulgarian] authorities round" to renounce his company's services.

Bulgaria froze the construction of South Stream on June 8, with outgoing Prime Minister Plamen Oresharski making public the halt of the project just minutes after meeting three US senators. His actions gave ground to claims that Washington had force Sofia into stopping activities.

The Russian businessman, whose stake at Stroytransgaz is about 63 percent and who has been imposed US (but not EU) sanctions over the Ukraine crisis, also reminded that Centrgaz's executive board had approved that the company should open a Bulgarian filial briefly after it won the tender to build South Stream in Serbia.

Reports of Centraz's intentions to set up an office in Bulgaria did emerge, but the company stopped short of making its plans public and did not reveal the Board of Directors's decision on its website.

Stroytransgaz, for its part, sparked controversy in Bulgaria this spring after reports revealed as early as March that it was to win the tender later (and it did in May) amid suspicions of flawed public procurement procedures.