Speaking from Afghanistan, where he is visiting German forces and development workers, Steinmeier said Germany would stand by Turkey.

He called for those responsible for Sunday night's attack to be brought to account.

The bombs hit the Istanbul suburb of Gungoren. Officials said people came out into the streets after a loud blast, which was then followed by another larger explostion 10 minutes later.

Sixteen people were killed and about 150 injured, the state-run Anatolian news agency reported.

No one has yet claimed responsibility for the blasts, the worst attacks in Istanbul since 2003. Turkish newspapers have reported that police are focusing their investigation on the Kurdistan Workers Party (PKK), saying it had used similar explosives in the past.

Bombs coincide with AK Party case

The bombings came just hours before Turkey's highest judicial body, the Constitutional Court, was set to begin deliberations on whether to shut down the country's ruling AK party. The party stands accused of seeking to introduce Islamic rule in Turkey, which although mainly Muslim, is officially a secular state.

The case has thrown the country in political and economic uncertainty as both local and EU politicians, as well as foreign investors, await the verdict, which analysts say could come by early August.