Turkish authorities have arrested four individuals in Istanbul in connection with a bomb attack targeting security forces in the city that killed 11. Turkey's Anadolu agency reported the four were arrested by anti-terror police following the explosion during rush hour in Istanbul's Vezneciler district on 7 June.

Seven of those killed in the attacks were members of the Turkish security forces, after the assault appeared to target a bus carrying riot police in the historic area of the city.

Istanbul Governor Vasip Sahin said at least three of the wounded were in a serious condition, according to the Associated Press.

The injured were taken to nearby hospitals, local media reported, as businesses in the vicinity of the bomb blast, which had suffered some superficial damage, were evacuated.

There were reports of gunfire in the historic Istanbul district in the immediate aftermath of the blast. In response to a perceived ongoing threat, police have set up a perimeter in the surrounding area, closed it to traffic and shut nearby metro stations.

Bomb disposal units were called to Vezneciler over fears of a secondary explosion. A suspicious white van was said to have been seen nearby to where the explosion took place.

While no individual group has claimed responsibility for the blast, Turkey has suffered a series of attacks from Islamist militants and Kurdish separatists in recent months. Islamic State (Isis) claimed two suicide bombings in tourist areas of Istanbul that killed 10 and four people in January and March respectively.

Kurdish militants usually favour military or police targets, although a splinter group of the Kurdistan Worker's Party (PKK), known as Kurdistan Freedom Hawks (TAK), was behind two recent bombings with high civilian casualties in Ankara.

In the wake of this morning's attacks, Turkish authorities have placed a blackout on distribution of any material concerning the aftermath and investigations into the blasts.