The report in daily El Pais, which cited government sources, was the first to put a figure on the amount China planned to commit to Spain, and said it was as much as Beijing would spend on Greek and Portuguese debt combined.

Officials from both countries would not confirm the figure or say when or what type of Spanish public debt China plans to buy.

Chinese Vice Premier Li Keqiang reaffirmed a commitment to buying Spanish bonds on Wednesday during an official visit in Madrid, but gave no details.

Li was embarking on a three-country European tour as markets watched for signs that China might step up its contribution to efforts to shore up Europe's finances, notably on the fiscally troubled euro zone periphery.

Spain's debt markets were little changed on Thursday, with the spread between Spanish and German ten-year bonds widening marginally to 243 basis points at 1135 GMT, having traded around 240 basis points for most of the morning.

"In the greater scheme of things, 6 billion would be a drop in the ocean for China. Nevertheless, the lack of reaction in the debt markets at the moments reflects more the reticence of investors to take the report seriously," 4Cast economist Jo Tomkins said.

A commitment of 6 billion euros would cover a fair chunk of Spain's projected issuance for 2011.

It plans to issue net medium and long-term debt for 47.2 billion euros in 2011, and expects to have about 540.8 billion euros of total public debt, or 54 percent of its GDP, in circulation by year end, the government said at the end of December.

Beyond vague statements promising support, however, China has revealed nothing about how much sovereign debt it holds in euro zone countries or how much its investment funds intend to buy.

The Spanish government would not say how much of its debt is in Chinese hands, but El Pais estimated that China's central bank has nearly 20 percent of the amount held by non-Spanish investors, who typically buy around 60 percent of the debt on offer.

French business daily La Tribune estimated on Wednesday that China held 7.3 percent of the total 8.861 trillion euros in outstanding euro zone public debt last June, based on extrapolations from Chinese currency reserve figures.

China has been increasing its holdings of European government debt, including that issued by Spain, Vice Commerce Minister Gao Hucheng was quoted separately as saying on Thursday.

In a statement on the Commerce Ministry's website, Gao also said that China was confident in Spanish and European financial markets and that it was also confident that they would be able to overcome Europe's debt crisis. Gao accompanied Li on his trip to Spain this week.

Li is next due in the United Kingdom and Germany.

Spain has come under increasing pressure from international debt markets on concerns it may be forced to follow Greece and Ireland and seek an EU or International Monetary Fund bailout. But while bond yields have risen, demand for Spanish debt remains solid.

Beijing in turn is under domestic political pressure to invest its foreign reserves carefully after suffering losses during the global financial crisis."