A demonstrator runs to take cover as Turkish riot police use tear gas to disperse Kurdish demonstrators during a protest against a curfew in Sur district and security operations in the region, in the southeastern city of Diyarbakir.

Four people were killed in clashes between security forces and Kurdish militants on Tuesday, security sources said, as fighting widened in southeast Turkey following a suicide bombing that killed 37 people in the capital Ankara.

Kurdistan Workers Party (PKK) fighters blocked roads and halted vehicles in the Kaynartepe neighborhood of the city of Diyarbakir and clashed with security forces sporadically through the night as a police helicopter flew overhead, witnesses said.

No one has claimed responsibility for Sunday’s car bomb that tore through a crowded transport hub in Ankara, but security officials have said it involved two militants, one of them female, from the outlawed PKK.

Violence has surged in mainly Kurdish southeast Turkey since a 2-1/2 year PKK ceasefire collapsed in July. The militants have focused their strikes on security forces in southeastern towns, some of which have been under curfew.

One police officer and three militants were killed in the fighting in the Baglar district of the southeastern city of Diyarbakir, the security sources said.

A curfew was imposed in Baglar’s Kaynartepe neighborhood from 3 a.m. (0100 GMT) after moves by militants to set up barricades, dig ditches and plant explosives, authorities said.

The curfew was later widened to encompass more city streets as clashes continued in the morning. Gunfire and explosions rang out across the city and police in armored vehicles parked on street corners called for people to stay inside.

Conflict in Diyarbakir, the southeast’s largest city, has until now has been focused in the Sur district, parts of which have been devastated by the fighting.

Also on Tuesday, Turkish police shut Istanbul’s Bosphorus Bridge, a key transport link spanning Asia and Europe, to traffic and were searching a suspicious vehicle, broadcasters CNN Turk and NTV reported.

Live footage of the bridge showed a series of white vehicles parked behind another lone white sedan near the European exit. Bomb disposal experts were near the vehicle, CNN Turk reported.

Related articles across the web

• Car bomb explosion in Turkish capital
• Bulgarian Foreign Minister denies Turkey is expelling consul from Istanbul
• Erdogan's War Against Kurds in Southeast Turkey is 'Part of Business Plan'
• Turkish PM: Those who attack 'will pay the price'
• Travels in Turkey: Part 4 - Istanbul, Continued: Great Bazaars and Grand Mosques
• Turkey: The Istanbul Bombing - Cui Bono - who benefits ?
• Qatar- Turkey's Erdogan says despite denials Syrian Kurdish PYD behind Ankara attacks
• Six killed in PKK attack on military convoy in southeast Turkey: Sources
• Turkish PM: Syrian regime responsible for Ankara attack
• Bomb in southeast Turkey kills seven security force members: sources