German police have arrested three men they suspect of preparing a serious act of violence against Jewish targets in Germany for Hamas, a week before the second anniversary of the militant Islamist group's attack on Israel.

Prosecutors suspect the three men of being foreign operatives for Hamas and of being involved in procuring firearms and ammunition to be used for assassinations targeting Israeli or Jewish institutions in Germany.

"In the course of today's arrests, various weapons, including an AK-47 assault rifle and several pistols, as well as a considerable amount of ammunition, were found," the federal prosecutors said in a statement on Wednesday.

The three, identified in line with German privacy laws only as German citizen Abed Al G., Wael F. M., born in Lebanon, and German citizen Ahmad I., were arrested in Berlin on Wednesday. A security source said the three were in their 30s or 40s.

Hamas, which has been running Gaza and is backed by Iran, is designated as a terrorist organization by the European Union and a number of other countries including the United States.

Hamas on Wednesday denied links to the three men in this case, calling the allegations unfounded and saying its struggle is "confined to opposing Israeli occupation in Palestine."

Separately, in February, four Hamas members suspected of plotting attacks on Jewish institutions in Europe went on trial in Berlin in what prosecutors described as the first court case against militants of the Palestinian group in Germany.

Anti-terrorism investigators observed the accused meeting in Berlin for a weapon handover before operational forces intervened and discovered functional weapons, including a Glock pistol, said a spokesperson for the prosecutor's office.

Forensic technicians are examining the arsenal and searches have also taken place in the eastern city of Leipzig, where one of the suspects lives. Authorities also conducted a search in Oberhausen in the western state of North Rhine-Westphalia, the spokesperson added.

Germany is one of Israel's strongest allies due to the legacy of the Holocaust and security is tight at synagogues and other Jewish institutions. It did not join France, Britain and several other countries last month in defying Israel's wishes and recognising Palestinian statehood.

Hamas has not yet responded to a 20-point plan for Gaza by U.S. President Donald Trump, which includes disarmament of the militant group.