There has been another exchange of fire across the border between Turkey and Syria, with Turkey's prime minister warning the two nations are "not far" from war.

In a belligerent speech to a crowd in Istanbul on Friday, Turkish prime minister Tayyip Erdogan warned the Syrian government it would be making a fatal mistake if it picked a fight with Turkey.

The heated rhetoric came as Turkey's military again returned fire across the border after Syrian shells again landed on Turkish soil on Friday.

The cross-border shelling follows a Syrian mortar barrage on a town in south-east Turkey that killed five people on Tuesday.

Turkish artillery continued to bombard Syrian military targets on Wednesday and Thursday in response, killing several Syrian soldiers.

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The Turkish parliament then authorised a one-year mandate for cross-border military action in the event of further aggression.

Mr Erdogan warned Syria that it would pay "a big price" for further attacks.

"We are not interested in war, but we're not far from it either," Mr Erdogan said in his speech.

"Those who attempt to test Turkey's deterrence, its decisiveness, its capacity - I say here they are making a fatal mistake.

"When they say 'if you want peace prepare for war' it means that when the time comes, war becomes the key to peace".

The head of Syria's main opposition coalition, the Syrian National Council (SNC), says president Bashar al-Assad is trying to turn the civil war into a regional conflict.

"This incident was a Syrian regime plan," SNC chief Abdel Basset Sayda told reporters at a press conference in Istanbul.

"The regime is trying to export the Syrian crisis. He (Syrian president Bashar al-Assad) thinks, that way, he can transform the conflict into a regional one."

Turkey, once an Assad ally but now a leading voice in calls for him to quit, shelters more than 90,000 Syrian refugees in camps on its territory and has allowed rebel army leaders sanctuary.

Violence has also spilled over into Lebanon.

The UN Security Council condemned the original Syrian attack and demanded that such violations of international law stop immediately.

The United States has said it stands by its NATO ally's right to defend itself against aggression spilling over from Syria's war.

Homs bombed

In the Syrian civil conflict, the military has launched an intense attack on the city of Homs.

The city has been an important centre of resistance and opposition activists say the government has launched its most intense attack there in five months.

Activist say rockets and bombs fell on the city killing eight civilians and a rebel fighter.

The London-based Syrian Observatory for Human Rights says six civilians and eleven rebels were killed in Aleppo, taking Friday's nationwide death toll to 95.

Meanwhile, Syrian rebel fighters are threatening to execute dozens of Iranians they kidnapped in August unless Syria's army withdraws from the embattled eastern Ghuta area of Damascus province.

It is the second time the rebels have issued such a threat against the Iranian hostages.

On August 5, rebels posted a video of 48 Iranians kidnapped in Damascus, saying they were elite Revolutionary Guards.

"We gave the regime 48 hours starting yesterday to withdraw completely from the eastern Ghuta area," said Abul Wafa, the commander of the rebels' so-called Revolutionary Military Council.

"We also have other secret, military demands. If the regime does not fulfil them we will start finishing off the hostages."

Sorry, this video has expired Dr Ben Macqueen from Monash University discusses Turkey, Syria relations

ABC/AFP