Turkey has shut its side of the last border crossing with Syria still controlled by President Bashar al-Assad's government, stepping up security following two deadly bombings this month.

Fifty-one people were killed when twin car bombs ripped through the Turkish border town of Reyhanli in the southern province of Hatay on May 11, heightening fears that Syria's civil war was dragging in neighbouring states.

Turkey has accused Syria of involvement in the attacks. Damascus has denied any role.

Customs Minister Hayat Yazici said the Yayladagi gate, some 90 km (55 miles) from Reyhanli, would remain closed for a month, during which only Turkish citizens arriving from Syria or non-Syrians transiting through Turkey would be allowed to cross.

Nobody would be allowed to cross from Turkey into Syria.

The gate was initially shut the day after the bombings to prevent the attackers from fleeing to Syria.

A Turkish official in the region said there would now be time to station bomb-detecting equipment at the crossing.

The Yayladagi gate is the only crossing along the 900 km shared border whose Syrian side is still controlled by Assad's government. All other crossings have fallen into the hands of rebels fighting to overthrow him.

The Yayladagi crossing serves as the main route to Syria's northwestern coastal area around Latakia, which has a large Alawite population. Assad is also Alawite, a minority sect that has dominated Syria for decades.

Turkey says it has detained 18 Turks in connection with the car bombings, 12 of whom are facing formal charges, and that among them were some of the main perpetrators, including the owner of the vehicles used in the attacks.

Officials said some of the perpetrators had travelled close to Yayladagi after the bombings to try and slip into Syria.

The bombings were some of the deadliest in Turkey's modern history, although violence from Syria's civil war, now in its third year, had spilled over the border before.

At least 14 people were killed in a bombing at the Cilvegozu border gate near Reyhanli in February. In October, a mortar fired from Syria killed five Turkish civilians in the frontier town of Akcakale further to the east.- Reuters