The latest blasts coincide with political tensions in Turkey
Three people have been slightly hurt in explosions by a municipal building in Istanbul, officials and media say.
Turkey's Anatolia news agency quoted a local mayor as saying there had been three blasts. Reports suggest mortars or hand grenades may have been used.
The attack took place in Uskudar, on the Asian side of the city.
Last month, double bombings blamed on Kurdish separatists killed 17 people in the city. The Kurdistan Workers' Party (PKK) denied any role in the attacks.
A month ago, three police officers and three gunmen were killed in an attack on the entrance to the US consulate in Istanbul. Turkish police said they were investigating possible links to al-Qaeda.
Police are searching for two suspects who are thought to have fled the scene of Thursday's explosions on a motorbike.
Local media reports initially suggested the blasts had been caused by hand grenades being thrown at the building.
But later TV reports, quoting security sources but unconfirmed by officials, say four mortars may have been fired from a neighbouring cemetery.
Uskudar mayor Mehmet Cakir told Anatolia that one blast had occurred in a rubbish truck in the car park of the municipal building and two more in the graveyard.
He said the explosions had caused minor damage to the building and to some vehicles parked nearby.
Istanbul police chief Celalettin Cerrah, speaking at the scene, said three workers had been slightly injured and that police were still investigating the cause of the blasts.
The scene outside the building in Istanbul
The latest incident comes at an uneasy time in Turkey, says the BBC's Sarah Rainsford in Istanbul, following the double bombing less than two weeks ago.
Last week, Turkey's Constitutional Court narrowly voted not to close down the governing AK Party, accused of undermining the country's secular system.
Meanwhile, an investigation continues into a shadowy ultra-nationalist group, known as Ergenekon, which is suspected of plotting to overthrow the government.
Dozens of people have been arrested and charged in connection with the inquiry, including two retired high-ranking military generals.
Turkey has seen armed attacks from a variety of groups in recent years.
The most deadly was in November 2003, when 58 people were killed by Islamist militants in suicide bombings outside two synagogues, the British consulate and a British bank in Istanbul.
The Kurdish rebels of the PKK have also been blamed for several attacks, including a car bombing that killed six people in the city of Diyarbakir in January.
Leftist and ultra-nationalist groups have also been accused of violence.