Turkey is on edge after the shooting rampage at the Reina nightclub unleashed shortly after revellers rang in 2017 which killed 39 people and was claimed by the Islamic State group.
A top official said the gunman may be a Turkic Uighur and several people of Uighur origin were arrested earlier on Thursday.
Just four days after the nightclub carnage, a car bomb exploded outside a courthouse in the Aegean city of Izmir on Thursday afternoon, with authorities blaming the outlawed Kurdistan Workers Party (PKK).
State-run Anadolu news agency said a policeman and a court worker, reportedly a bailiff, were killed.
Police battled "terrorists" in a clash which saw two militants killed while another escaped and is now being pursued.
Several ambulances rushed to the scene after the blast in the usually peaceful port city, Turkey's third largest metropolis, and the gateway to the plush beach resorts of the Aegean.
There has so far been no claim of responsibility for the latest bombing after a year of bloodshed in the Muslim-majority NATO member.
But Izmir governor Erol Ayyildiz said that initial evidence suggested the PKK -- which has fought a deadly insurgency for over three decades -- was behind the attack, rather than IS.
He said police tried to stop the car before it exploded and the "terrorists" then sought to escape as the explosion went off and the gunfight began. Up to seven people were wounded, he added.
Usually calm Izmir lies well to the west of the PKK's main theatre of operations in the Kurdish-dominated southeast of the country.
'Uighur origin'
Turkish authorities meanwhile were seeking to close in on the Istanbul club attacker, who slipped into the night after spraying 120 bullets at terrified partygoers celebrating New Year.
Of the 39 dead, 27 were foreigners including citizens from Lebanon, Saudi Arabia, Israel, Iraq and Morocco.
A top official said the attacker was likely a Turkic Uighur and reports have indicated the authorities are looking into the possible existence of a cell, also including other jihadists from Central Asia.
IS took responsibility for the massacre in a statement on Monday, the first time it has issued a clear and undisputed claim for a major attack inside Turkey.
The extremist group said it was a response to Ankara's military operation against the jihadists in northern Syria, where Turkish armed forces are supporting opposition fighters retaking territory from IS.
Deputy Prime Minister Veysi Kaynak told A Haber broadcaster on Thursday that the attacker was "probably" of Uighur origin as he sought to play down fears that the gunman would escape Turkey.
Most Uighurs, an eastern Turkic group, live in the Xinjiang region of China, although there are also significant populations in ex-Soviet Central Asian states.
Previous reports had said the killer could be from Kyrgyzstan or Uzbekistan.
Kaynak said airports had taken measures to ensure the killer did not flee Turkey and Dogan reported that the authorities also tightened land borders.
The agency said checkpoints would be set up to search all vehicles and people leaving the country at border crossings in Edirne, western Turkey, which has a land border with Greece and Bulgaria.
"The security forces are aware of who the terrorist is. We are also aware of the probable place where he might be," Kaynak added, without giving further details.
Kaynak described the massacre as "sophisticated and well planned", suggesting the gunman is part of a "well formed cell".
Families detained in Izmir
Special forces detained several people Uighur origin suspected of links with the Istanbul attack on the outskirts of the city on Thursday, Anadolu said.
Excluding the latest arrests, at least 36 had been detained in the probe by Wednesday.
Authorities had on Tuesday detained 20 adults -- along with 20 of their children -- from three families in Izmir. There was no indication of any link to the bombing in the city.
Those held had moved to Izmir on December 10 from the central city of Konya, where they are suspected of having been in touch with the killer during his stay there.
Police are investigating possible links to IS of the suspects, who include Uighurs, Kyrgyz citizens and suspects from the Russian Caucasus region of Dagestan.
The latest attack comes after Turkey suffered a bloody year in 2016 when multiple bombings blamed on Kurdish militants and IS jihadists killed hundreds of people.
