Turkey's military has fired an artillery round into Syria in immediate retaliation after a shell fired from Syria landed in the Turkish border town of Akcakale – the second such incident in five days.

Prime Minister Tayyip Erdogan warned Syria on Friday Turkey would not shy away from war if provoked, but a series of mortar bombs fired from Syria have hit Turkey since then.

There were no casualties when the latest Syrian shell hit land near a plant belonging to the Turkish Grain Board (TMO), several hundred feet from the centre of Akcakale, where five civilians were killed on Wednesday in previous Syrian shelling.

The exchanges are the most serious cross-border violence in Syria's conflict, which began as pro-democracy protests but has evolved into a civil war with sectarian overtones.

Nato member Turkey was once an ally of Syrian President Bashar al Assad but turned against him after his violent response to an uprising in which, according to the United Nations, more than 30,000 people have died.

The Syrian Observatory for Human Rights said the shells fired from Turkey landed near the Syrian town of Tel Abyad.

Broadcaster NTV said the shell from Syria landed in the garden of the TMO plant near storage silos. It said the silos had suffered some damage from shrapnel.

Akcakale had been quiet since Turkey's retaliation on Wednesday and Thursday for the initial shelling, but Syrian government forces began shelling areas around Tel Abyad yesterday morning.

Before the latest strike into Turkey, the Syrian military had fired seven artillery shells yesterday into an area close to the Syrian customs building, which is around 300 metres from the border and under the control of rebel forces.

People were reportedly killed in those strikes and two Syrians wounded were carried through the border fencing and taken to a hospital in Akcakale.

Yesterday evening a car bomb exploded near the police headquarters in Damascus. Residents reported hearing a huge explosion in the Fahameh district that rattled the city just after sunset.

Syrian TV described the blast as a "terrorist" attack.

In a further development, Syrian rebels have seized a government army outpost near the Turkish border province of Hatay.

The rebels took control of the three-storey white building, around half-a-mile from the border on a hill overlooking the Turkish village of Guvecci on Saturday, and raised the flag of the Free Syrian Army.

"In the last four days there were heavy clashes going on here. We couldn't sleep. Yesterday morning, the Syrian army controlled this area. Now it is calmer," said villager Musa Sasak, 27.

Three mortar bombs fired from Syria landed near Guvecci on Saturday, prompting a fourth day of retaliatory fire from Turkish forces. The Syrian mortar rounds hit empty land and there were no casualties. The exchanges highlight how the crisis could destabilise the region.

Clashes could be heard yesterday in the area of the Syrian village of Khirbet al Joz, behind the hill where the military outpost was located. Smoke could be seen rising from the area.

The Syrian Observatory for Human Rights said rebels seized control of Khirbet al Joz and the land around it late on Saturday after a 12-hour battle.

The British-based group said at least 40 Syrian soldiers, including five officers, were killed. Nine rebel fighters also died, it said.

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.