Iran's seizure of a UK-flagged tanker in the Strait of Hormuz has further heightened tensions in the Gulf region after the US military claimed it had downed an Iranian drone.

The far-right Alternative for Germany, battling stagnant poll results and suspicions of illegal funding, is now torn by a deepening rift between its party leadership and its most radical wing.

The turbulence comes as falling migrant arrivals have deprived the ultra-nationalist protest party of its main rallying cause, while the ascendant Greens have replaced them as the strongest opposition force.

Berlin said Monday it had no plans to send ground troops to Syria, refusing a US request for Germany to ramp up its military involvement in the fight against IS militants.

"When I say that the government intends to continue with its ongoing measures in the framework of the anti-IS coalition, then that means no ground troops," said German government spokesman Steffen Seibert.

A 100-kilogramme (220-pound) US World War II bomb was defused overnight near Berlin's central hub of Alexanderplatz, police said Saturday.

Iran's conservative press Tuesday mocked the EU's position on a 2015 nuclear deal abandoned by Washington, accusing Germany's foreign minister of exhibiting both impotence and high-handedness during talks in Tehran.

Since US President Donald Trump quit the agreement in May last year, the European Union has repeatedly renewed its own commitment and pledged to reward Iran's compliance with the sanctions relief promised under the deal.

German Chancellor Angela Merkel's favoured successor Annegret Kramp-Karrenbauer sparked outrage on social media on Tuesday with an apparent call for limits to free speech around elections. 

Germany's government commissioner on anti-Semitism has warned Jews about the potential dangers of wearing the traditional kippah cap in the face of rising anti-Jewish attacks. 

"I cannot advise Jews to wear the Kippah everywhere all the time in Germany," Felix Klein said in an interview published Saturday by the Funke regional press group.

Germany has stopped processing some applications for asylum by Syrian refugees, pending a new assessment of the security situation in the war-torn country, according to a report on Saturday.

Greece's parliament on Wednesday began debate on a resolution to demand the payment of German war crime reparations, an issue long disputed by Berlin.

"These demands are always active. They were never set aside by Greece," parliament chairman Nikos Voutsis told reporters this week.

The German government has extended a ban on arms exports to Saudi Arabia by six months until the end of September, but is making a conditional exception for systems developed jointly with other countries.

Germany imposed the ban following the killing of Saudi journalist Jamal Khashoggi at the kingdom's consulate in Istanbul last year. Britain and France have criticized Germany's stance, saying the ban prevented them selling jointly developed equipment with German components to the Gulf nation.