GUVECCI, Turkey — Turkey closed its border with Syria on Wednesday in an attempt to hold back the chaos and lawlessness that has spread along the border, as Syrians flee the intense fighting between rebels and the army of Syrian President Bashar Assad.
Meanwhile, fierce battles raged in Aleppo, Syria's largest city, for the fifth day. Human rights activists said 32 people died in the fighting.
Mr. Assad was hit with more diplomatic defections, as his ambassador in Cyprus and her husband, a former ambassador to the United Arab Emirates, left their posts.
Syrians fleeing to the Turkish border, about 40 miles from Aleppo, reported that they had been targeted by thieves taking advantage of the insecurity in what analysts called a "no man's land."
"I can't do anything. Nobody can help me. There's no government," said Dibo Sezer, a Syrian businessman.
He said he was transporting cars into Turkey when his two trucks were burned and his cargo snatched as he crossed the border at the Bab al-Hawa checkpoint.
"We were stopped and asked who we are and where we are going, but we didn't know if these were rebels, thieves, government soldiers or what," he said this week while standing next to his charred vehicles.
The border area along Turkey has become increasingly perilous, as regime forces clash with rebel fighters who have gained control of three border crossings during the past week.
"We have serious concerns over the safety of Turkish trucks regarding their entry and return from Syria," said Turkish Economy Minister Zafer Cagalayan, although he added that refugees still would be allowed into the country.
Border crossings had allowed $3 billion in annual trade between the countries.
Turkish authorities are becoming increasingly concerned that the conflict could spill across the border.
Kurdish parties that oppose Turkey control parts of the north of Syria, and the war zone on the border is expected to heat up further in the coming weeks. Turkey is a NATO member and a major U.S. ally in the region.
"Turkey is finding itself in a very unpleasant and dangerous situation," said Fawaz Gerges, director of the Middle East Center at the London School of Economics.
"If the Turkish-Syrian border becomes any more inflamed and if the situation escalates, I doubt very much whether Turkey can really shield itself. The Turkish leadership is very much concerned about terrorism striking at the heart of Turkish cities. That's really the nightmare scenario for Turkey."
Since the assassination of key figures in the Assad regime a week ago, fighting has spread from the capital city of Damascus to Aleppo, where regime and opposition forces have clashed during the past several days.
Regime forces have been using helicopters and fighter jets to crush the opposition. On Wednesday, it was reported that thousands of regime troops were being redeployed from the border regions to engage in fighting in the two cities.
Turkey, on Syria's northern border, was an ally of Mr. Assad before his forces started their brutal crackdown on demonstrations 16 months ago. The country has since sheltered Free Syrian Army rebels as well as thousands of refugees fleeing the violence.
"Every night, we cross to Syria, collect the injured, bring them to Turkey and get them to a hospital," said Yassir, a rebel who gave only his first name.
"Turkey is being very easy with us. They let us take food across to the rebels and bring ambulances to the border to collect the injured."
Turkey, which also has hosted the civilian opposition Syrian National Council in Istanbul, is not the only country that has provided support to the Free Syrian Army, analysts said.
"Arms are beginning to flow particularly from the Gulf states," said Mr. Gerges. "We also know that the Americans and Western powers are providing intelligence, nonlethal equipment, communication equipment, logistics that provide the rebels with command and control."
But along the chaotic border, rebel fighters said they had not been receiving help and had been forced to retreat from clashes inside Syria as they ran out of ammunition and food.
"After we ran out of bullets, there was nothing we could do," said rebel battalion commander Abu Salam. "We had no choice but to cross the border."
Like many other rebel fighters embroiled in the conflict, Mr. Salam and his comrades are frustrated by the lack of funding and help for the opposition forces.
"We haven't received any help or assistance from the opposition politicians, the Syrian National Council or anyone," Yassir said. "They're taking the money and putting it in their pockets, staying in five-star hotels in Istanbul, while we are here with barely enough food."
• Louise Osborne, reporting from Berlin, contributed to this article, which is based in part on wire service reports.