German Chancellor Angela Merkel Friday defended her government's silence on a reported secret deal to sell hundreds of tanks to Saudi Arabia, and said she was committed to democracy in the region.

"Deliberation and decisions by the federal security council are secret for good reason," she told the daily Mittelbayerische Zeitung, referring to the panel including the chancellor and top ministers that rules on arms exports.

Saudi Arabia is reportedly about to buy 200 Leopard-2s, Germany's main battle tank which is also produced under licence in Spain, for a multi-billion-euro sum.

Germany, which for two decades has declined to sell such heavy weapons to Saudi Arabia because of concerns over human rights and fears for Israel's security, has refused to officially confirm the reports citing a secrecy policy on such deals.

Opposition politicians and even members of Merkel's ruling centre-right coalition have slammed the reported tank sale, particularly in light of democratic uprisings throughout the Middle East.

Selling tanks to Saudi Arabia at a time when that country has sent armoured vehicles to help put down a peaceful protest movement in neighbouring Bahrain is "a slap in the face for freedom movements in the whole region," Social Democrat parliamentary deputy leader Gernot Erler said this week.

Merkel insisted in the interview that her administration was "of course doing its part to continue to support democratic development in North Africa and the Middle East together with our partners".

When asked about criticism of Berlin's secrecy on a delicate issue, she said her administration was following official guidelines.

"Transparency about exported weapons and other armaments is assured because every year a detailed arms export report is published which is also given to the Bundestag" lower house of parliament, she said.

However a leading deputy from the Free Democrats (FDP), junior partners in Merkel's coalition, said the government should go on the offensive now that the country was openly debating the issue.

"It damages the government and it damages Germany too when only those who oppose (the sale) are heard," the foreign policy spokesman of the FDP's parliamentary group, Rainer Stinner, told the daily Rheinische Post.

"The chancellor and the affected ministers cannot keep hiding behind the sign reading 'secret'."

Opposition deputies were to present motions to the Bundestag Friday demanding Berlin call off the deal.

The Green party said it would file a lawsuit against unnamed executives at the tank manufacturer Krauss-Maffei Wegmann in a move to force the German government to shed light on the matter.

A parliamentary whip, Volker Beck, told the daily Sueddeutsche Zeitung that the suit was based on suspicion that selling the tanks to Riyadh would violate arms export laws.