BERLIN (Reuters) - The European Commission fears steps taken by Volkswagen to refit polluting diesel cars may damage the vehicles' engines, the Spiegel magazine reported, citing unidentified staff at the European Union's executive branch.

Software updates carried out by Volkswagen (VW) could inflict greater stress on engine components, Spiegel cited a member of staff at the Brussels-based Commission as saying.

Commission officials are basing their concerns on feedback from experts at the "Vela" emissions laboratory in northern Italy, the weekly magazine said, without providing more detail.

The Commission was not immediately available to comment.

Germany-based VW denied its technical modifications would cause damage to the cars.

"The software update will have no adverse influences on (fuel) consumption or the durability of the engine and its components," a company spokesman said.

EU consumer affairs commissioner Vera Jourova has sought to ramp up pressure on VW to compensate European owners of rigged diesel vehicles and has asked the carmaker for guarantees its technical fixes will work.

"We need VW to guarantee, in a legally binding way and without any time limit, that the repairs will work and do not have any negative impact," a Commission official told Reuters on Monday.

Consumer groups and politicians in Germany have been angered by VW's refusal to offer compensation, noting drivers are stymied in many EU nations by their inability to file U.S.-style class action lawsuits.

Germany's VZBV consumer lobby is backing the EU's call for VW to guarantee its technical fixes will have no negative effects on vehicles. But it made no progress in a meeting with VW on Friday to secure such a guarantee, a spokeswoman said.

"An advanced guarantee declaration is not necessary," a VW spokesman said, noting the carmaker's diesel fixes were subject to comprehensive tests to ensure there will be no negative impact on engine components or emissions clean-up systems.

Germany's KBA motor vehicle authority has so far cleared 5.6 million diesel cars affected by VW's emissions-test cheating scandal for repair. VW said on Friday the number of cars refitted had more than doubled to 1.23 million in recent weeks from 500,000 previously.

Approval by the KBA is valid for countries throughout Europe where 8.5 million cars of nearly 11 million vehicles worldwide are affected.

VW group models with 1.2-litre and 2.0-litre engines only require a software update on pollution control systems, whereas about 3 million 1.6-litre engines, besides the software update, also require a mesh to be installed near the air filter.

(Reporting by Andreas Cremer. Additional reporting by Alissa de Carbonnel.; Editing by Tina Bellon and Mark Potter)