ANTAKYA, Turkey (Reuters) - Until recently, Hamdi Esen would make the short trip across to Syria several times a month, fill his father's car up with gas, maybe buy a few bags of sugar and some cigarettes and then return home to Turkey.
But after Turkey stepped up criticism of Syria's crackdown on pro-democracy protesters, Esen stopped going because of long waits at the border and the hostility he faced from Syrian security officials and even some regular citizens.
"I used to drive over to Syria every week and fill up my tank. Gasoline is so much cheaper there. But now I don't go," said 31-year-old Esen as he sat chatting with friends at a roadside tea house.
"They treat us differently. It's as if they don't like us anymore."
Esen is from Antakya, the ancient city of Antioch, in Turkey's southern Hatay province, a panhandle that juts down into Syria and was once part of it.
With a Syrian mother and a Turkish father, Esen epitomizes the inhabitants of a frontier region where family ties transcend political borders and where people share a common history, culture and language.
Arabic flows as freely as Turkish on Hatay's streets as bilingual residents sit down to a plate of hummus, a staple dish in Syria and throughout the Arab world but rare in most parts of Turkey, though it is sometimes thought of as a Turkish dish.
In Harbiye, a holiday strip on the outskirts of Antakya, street vendors hang souvenir carpets of Syrian President Bashar al-Assad next to those of Turkey's founding father, Mustafa Kemal Ataturk, to sell to passing Turkish and Syrian tourists.
It is not surprising that the souring of ties between Turkey and its once-close ally is most palpable here.
After the collapse of the Ottoman Empire during World War One, Hatay passed to the French mandate of Syria which gained independence in 1936. But it became part of Turkey in 1939 and Syria's old claim to the area has plagued ties with its neighbor.
Having failed for months to persuade Assad to exercise restraint and introduce reforms in Syria, Turkey's Prime Minister Tayyip Erodgan has raised pressure on the Syrian leader and Turkey is now on the verge of imposing sanctions on a government it once considered a friend.
Turkey has allowed Syrian dissidents to meet in Turkish cities to form a political opposition to the Syrian government and is now giving sanctuary in Hatay to the most senior Syrian military officer to defect.
Adding salt to the wounds, Turkey this month conducted military exercises near the Syrian border in Hatay. Erdogan, who has said his patience with Assad has run out, is expected to visit Syrian refugee camps in the province soon.
All of this has had an effect on people like Esen who had become used to traveling unhindered back and forth across the border.
"We used to be able to travel across freely, but it's different now. Why? What have we done?" Esen asked.
In 2009, Turkey and Syria signed a visa waiver agreement for both its citizens and bilateral trade has boomed ever since, worth some $2.5 billion in 2010. Investments by Turkish firms in Syria last year also reached $260 million, Turkish data shows.
But trade is also informal. Lured by brand names and quality products, Syrians flock in their thousands to Turkey in search of a bargain, and in return Turks smuggle over cheap Syrian consumables such as sugar, cigarettes or gasoline -- products that are highly taxed in Turkey -- to sell on the black market.
"Farmers even bring cows over from Syria and then brand them here. Everything there is so much cheaper," said Hakan Celik, a 28-year-old hairdresser from Antakya.
"But it is much less now, fewer people are going across."
In September, Syria imposed an import ban on almost all consumer goods but quickly rescinded the embargo after a spike in prices and disquiet among the influential merchant class that has been backing Assad. Turkey's economy minister said last week trade with Syria was continuing at its normal pace.
In Reyhanli, a main border crossing in Hatay, Turkish trucks queue for hours in a line snaking back several kilometers (miles). Drivers reported no let-up in work but some said they now faced hostility from some Syrians and felt increasingly unsafe.
"They swear at our prime minister. They say 'we stamp on Erdogan with our shoes'," said 31-year-old truck driver Mehmet Ozdemir, who was transporting oven materials to Syria.
"They tell us our women are being raped in the Turkish camps and say 'you're taking them to Antalya and selling them'," Ozdemir said, referring to a city on Turkey's Mediterranean coast popular with Turkish and foreign tourists.
More than 7,500 Syrian refugees are living in six camps in Hatay and last month, Syrian state media reported that women were being raped and tortured in the camps. Turkey has strongly denied the claims.
"It is hard now to travel through Syria. We used to make our own food by the side of the road or sit in a restaurant but now we don't stop, we just drive straight through because of the security," said another truck driver, Abdulwahid Erisik.
"Sometimes they stop our convoys on the road. They threaten you with a gun. They wear civilian clothes. I don't know if they are police, civilians or what," said Erisik.
For some, the hostility has led to resentment of Turkey's assistance to the thousands of Syrians seeking refuge in Hatay.
"They are all gypsies, they don't have an ID card in Syria so they come here to get a Turkish passport," said hairdresser Celik, referring to the Syrian refugees.
Turkey does not refer to them as refugees, saying they are "guests" and can come and go as they please.
"We don't have a problem with the Syrian people but we should sort out our problems first," said another truck driver who did not want to be named.
Editing by Sonya Hepinstall