Thousands of people most likely will be massacred if Kobani falls to Islamic State fighters, a UN envoy said on Friday, as militants fought deeper into the besieged Syrian Kurdish town in full view of Turkish tanks that have done nothing to intervene.

UN envoy Staffan de Mistura said Kobani could suffer the same fate as the Bosnian town of Srebrenica, where 8,000 Muslims were murdered by Serbs in 1995, Europe's worst atrocity since World War Two, while UN peacekeepers failed to protect them.

"If this falls, the 700, plus perhaps the 12,000 people, apart from the fighters, will be most likely massacred," de Mistura said. The United Nations believes 700 mainly elderly civilians are trapped in the town itself and 12,000 have left the centre but not made it across the border into Turkey.

"Do you remember Srebrenica? We do. We never forgot and probably we never forgave ourselves," said de Mistura, the UN peace envoy for Syria. "When there is an imminent threat to civilians, we cannot, we should not, be silent."

The plight of mainly Kurdish Kobani has unleashed the worst street violence in years in Turkey, which has 15 million Kurds of its own. Turkish Kurds have risen up since Tuesday against President Tayyip Erdogan's government, which they accuse of allowing their kin to be slaughtered.

At least 33 people have been killed in three days of riots across the mainly Kurdish southeast, including two police officers shot dead in an apparent attempt to assassinate a police chief. The police chief was wounded.

Intense fighting between Islamic State fighters and outgunned Kurdish forces in the streets of Kobani could be heard from across the border. Warplanes roared overhead and the western edge of town was hit by an air strike, apparently by US-led coalition jets.

But even as the United States has increased its bombing of Islamic State targets in the area, it has acknowledged that its air support is unlikely to be enough to save the city from falling.

'TRAGIC REALITY'

"Our focus in Syria is in degrading the capacity of (Islamic State) at its core to project power, to command itself, to sustain itself, to resource itself," US Deputy National Security Adviser Tony Blinken said. "The tragic reality is that in the course of doing that there are going to be places like Kobani where we may or may not be able to be effective."

Blinken said Islamic State controlled about 40 per cent of Kobani. The Syrian Observatory for Human Rights, which monitors the war, gave a similar estimate and said fighters had seized a central administrative area, known as the "security quarter".

Ocalan Iso, deputy head of the Kurdish forces defending the town, told Reuters that Islamic State fighters were still shelling the centre, which proved it had not yet fallen.

"There are fierce clashes and they are bombing the centre of Kobani from afar," he said, estimating the militants controlled 20 per cent of the town. He called for more US-led air strikes.

In Washington, the US State Department said Turkey has agreed to support the training and equipping of moderate opposition groups in Syria and that a US military planning team would visit Ankara next week to further discuss the matter. The United States has been pressing Turkey to join the fight against Islamic State.

The Middle East has been transformed in recent months by Islamic State, a Sunni militant group that has seized swathes of Syria and Iraq, crucifying and beheading prisoners and ordering non-Muslims and Shi'ites to convert or die.

The United States has been building a military coalition to fight the group, an effort that requires intervening in both Iraq and Syria, countries with complex multi-sided civil wars in which nearly every state in the region has allies and enemies.

International attention has focused on Turkey, a Nato member with the biggest army in the region, which has absorbed 1.2 million Syrian refugees, including 200,000 from Kobani in the last few weeks. Erdogan has so far refused to join the military coalition against Islamic State or use force to protect Kobani.

"We would like to appeal to the Turkish authorities ... to allow the flow of volunteers, at least, and their own equipment in order to be able to enter the city and contribute to a self-defence action," the UN envoy de Mistura said in Geneva.

'FIGHT TO LAST BREATH'

The Kurdish uprising in Turkey provoked a furious response from the Turkish government, which accuses Kurdish political leaders of using the situation in Kobani to destroy public order in Turkey and wreck its own delicate peace process.

Turkish Kurds fought a decades-long insurgency in which 40,000 people were killed. A truce last year has been one of the main achievements of Erdogan's decade in power, but Abdullah Ocalan, jailed co-founder of the Kurdish militant PKK, has said the peace process is doomed if Turkey permits Kobani to fall.

In a televised speech on Friday, Erdogan accused Kurdish leaders of "making calls for violence in a rotten way".

"I have put my hand, my body and my life into this peace process," he said. "And I will continue to fight until my last breath to restore the brotherhood of 77 million at any cost."

The three days of riots in southern Turkey were the worst street violence in many years. The attempted assassination of a police chief in eastern Bingol province was the first incident of its kind since 2001. The armed wing of the PKK denied involvement in the attack.

The southeastern border province of Gaziantep saw some of the worst violence overnight, with four people killed and 20 wounded as armed clashes broke out between protesters calling for solidarity with Kobani and groups opposing them.

Footage showed crowds with guns, swords and sticks roaming streets of Gaziantep. Two local branches of the Kurdish People's Democratic Party (HDP), Turkey's main Kurdish party, there were torched, Dogan News Agency reported.

Many of Turkey's Kurds say the refusal to defend Kobani is proof the government sees them as a bigger enemy than Islamic State. At the frontier, dozens of Kurdish men watched Kobani's fighting from a hill where farmers once tended pistachio trees.

"I believed in the peace process, because I didn't want any more children to die. But the Kurds were fooled. The peace process was insincere. The government either wants to wipe out Kurds or to enslave them," said Ahmet Encu, 46, who came 500 km (300 miles) to watch Kobani, where four relatives are fighting.

Turkey says it would join an international coalition to fight against Islamic State only if the alliance also confronts Syrian President Bashar al-Assad's government. Erdogan wants a no-fly zone to prevent Assad's planes from flying over the area near its border and a protected buffer zone there for refugees.

The United States has said it is studying the idea but has made clear it is not an option for now.

The Pentagon said the top US military officer, General Martin Dempsey, will convene a meeting of more than 20 foreign defence chiefs next week outside Washington to discuss the multinational campaign against Islamic State.  – Reuters