Berlin tightened controls on foreign investments last year after a series of high-profile takeovers by Chinese firms, by extending a 25 percent shareholding threshold, at which the government can intervene, to additional business sectors.

Speaking to reporters in Berlin, Economy Minister Peter Altmaier said the government was looking into tightening those rules even further.

"The question of (lowering) the takeover threshold is one of many options on the table that we can discuss in the government and parliamentary groups," said Altmaier, one of the closest allies of centre-right Chancellor Angela Merkel.

Coalition sources have said the government could lower the foreign takeover threshold to 15 or 20 percent. German magazine Wirtschaftswoche reported on Thursday that Altmaier was open to lowering the threshold even further to 10 percent.

"I want Chinese companies to continue to invest here. But that must also be possible the other way around," Altmaier said, reflecting concerns that China is gaining too much access to key technologies in Germany and other countries while shielding its own companies from foreign takeovers.

"It's about fair economic relations and rules that have to apply to both sides. We also have a duty to protect our critical infrastructures," Altmaier said.

The comments follow an intense debate about Chinese investments in Europe, after a series of corporate takeovers in western Europe and infrastructure investments, notably in Greece and the Balkans.

The most prominent case in Germany so far was the purchase of German robotics maker Kuka by Chinese company Midea in 2016.

The debate has also been fuelled by Chinese carmaker Geely's move in February to acquire a stake of almost 10 percent in Germany's Daimler.

The head of Germany's domestic intelligence agency has called for vigilance over increased moves by Chinese companies to invest in and acquire high-technology German companies, warning the loss of key technologies could harm the German economy.

Altmaier's comments also come amid concerns over protectionism and a trade war after U.S. President Donald Trump decided to impose duties on steel and aluminium to counter cheap imports, especially from China.

In response to a request from Germany, France and Italy, the European Commission unveiled a proposal last year for a European investment screening mechanism, which is now being debated by member states.

(Additional reporting by Gernot Heller; editing by David Evans)

By Michael Nienaber

Stocks treated in this article : Kuka, Daimler, Midea Group Co Ltd