A group of hackers that cyber-security experts say targets critics of the Russian government has been trying since April to attack the computer systems of German Chancellor Angela Merkel's Christian Democratic Union party, a security research firm said on Wednesday.

Researchers at Trend Micro said the hackers, called Pawn Storm, appear to be trying to steal personal and corporate data from the CDU and high-profile individuals using two free email services. A year ago, the research group linked Pawn Storm to hacking attacks on the Bundestag, the lower house of parliament.

In the latest attempts, the hackers apparently tried to coordinate credential-phishing attacks, using computer services based in Latvia and the Netherlands, to gain access to the systems of the CDU and other high-profile users, Trend Micro said in a blog post.

"Up until now no attacks have taken place," a source at the CDU headquarters in Berlin said on Thursday. "We have nonetheless made appropriate changes to our IT infrastructure. We can't say anything on the reasons for this."

Pawn Storm has been active for more than a decade and is considered one of the longest-lasting cyber espionage groups.

Several major computer research groups say it has targeted opposition groups in Russia as well as NATO and governments in adjacent Eastern European countries, Turkey and the United States. The military, defense companies and media in those countries have also been attacked, Trend Micro said.

"Pawn Storm clearly targets groups that could be perceived as a risk to Russian politics and interests," Trend Micro said.

The cyber attack on the German parliament was first reported in May 2015. German media have said replacing the computer system could cost the government millions of euros.

The news magazine Der Spiegel also quoted an internal investigation as saying there were indications that a Russian intelligence agency had staged the attack.

In January 2015, German government websites, including Merkel's website, were hacked in an attack claimed by a group demanding that Berlin end support for the Ukrainian government.