Turkish forces fired across the frontier into Syria today after a shell launched from Syria landed in Turkey’s border town of Akcakale.

Turkey has warned that it will respond with force to any violence spilling over into its territory.

It was the fifth consecutive day of Turkish retaliation against incoming bombardment from northern Syria, where President Bashar al-Assad’s forces have been battling rebels who control swathes of land close to the Turkish frontier.

The exchanges are the most serious cross-border violence in Syria’s revolt against President Assad, which began in March last year.

The latest Syrian shell landed near a plant belonging to the Turkish Grain Board, the Dogan news agency reported, several hundred metres from the centre of Akcakale where five civilians were killed on Wednesday in previous Syrian artillery fire.

The Syrian Observatory for Human rights said shells fired in retaliation from Turkey today landed near the Syrian town of Tel Abyad, where rebels have been battling Mr Assad’s forces.

There were no immediate reports of casualties.

A Turkish border unit also returned fire after a mortar bomb launched from Syria hit an uninhabited area of Altinozu at the western end of the Syrian border, the Hatay province governor's office said.

It said the mortar bomb, believed to have been launched by Syrian government forces, did not result in casualties.

The violence continued inside Syria too, with state media and opposition activists saying a car bomb exploded near a police headquarters in the capital Damascus on Sunday.

State TV said one person was wounded. The Observatory said people had been killed and wounded but gave no details.

NATO member Turkey was once an ally of Mr Assad’s but turned against him after his violent response to the uprising in which activists say 30,000 people have died.

Turkey has nearly 100,000 Syrian refugees in camps on its territory, has allowed rebel leaders sanctuary and has led calls for Mr Assad to quit.

Its armed forces are far larger than Syria’s.

Prime Minister Tayyip Erdogan has said Turkey does not want war, but warned Syria not to test its resolve.

“You have to be ready at every moment to go to war if it is necessary. If you are not ready for this, you are not a state,” President Erdogan said in a speech in Istanbul.

“What did our forefathers say? ‘If you want peace prepare for war’ ... If you suffer an outrage, you will do what is necessary. Look, we are retaliating in kind,” he said.

“If you strike, you will see the retaliation immediately.”

Damascus has said its fire hit Turkey accidentally.