Turkish police on Tuesday shot dead a man and a woman belonging to a leftist "terrorist" organisation who attacked a security checkpoint outside Istanbul's main courthouse, injuring six people, officials said.

Interior Minister Ali Yerlikaya said the assailants were members of the Revolutionary People's Liberation Party-Front (DHKP-C) -- a fringe leftist group that has staged periodic attacks in Turkey since the 1980s.

The group issued no initial claim of responsibility.

"While the terrorists who attempted to attack were neutralised, six people, including three police officers and three citizens, were injured," Yerlikaya said in a social media statement.

Justice Minister Yilmaz Tunc said "heroic police officers prevented a treacherous attack", adding that prosecutors had launched a "multi-faceted investigation".

Images from the scene showed two bodies lying on the ground outside the main entrance of the Caglayan courthouse, which has been used for some of Turkey's biggest trials.

Police sealed off the courthouse entrances as a security precaution.

Turkey has begun to emerge from a violent spell which started a decade ago, when it was hit by repeated bombings and other attacks linked to jihadist fighters and Kurdish militants.

Although those attacks have largely died down, both Istanbul and the capital Ankara remain on high alert.

Last month, one man was shot dead by two gunmen who opened fire inside a Catholic church in Istanbul.

The attack was claimed by Islamic State group jihadists.

In October, two assailants injured two policemen in an attack on the government district in the capital Ankara that was claimed by Kurdish militants.

Turkey responded by stepping up air strikes against Kurdish targets in Syria and Iraq.

In one of its highest-profile attacks, the DHKP-C in 2013 staged a suicide bombing of the US embassy in Ankara, killing a Turkish security guard.

The group, which is recognised as a terrorist organisation by Washington, has been fighting US influence in the Middle East and across the world.

In 2014, Washington issued a $3 million reward for the capture of the group's leaders.