Thick smoke rises from burning vehicles at the site of a car explosion on the Syrian border crossing of Bab al-Hawa. (Reuters)

A car bomb exploded at a major crossing between Syria and Turkey on Tuesday, killing seven people, as Syria’s civil war continued to spill beyond its borders amid a recent upsurge in violence.

The afternoon bombing at the Bab al-Hawa crossing came a day after Turkey shot down a Syrian helicopter that it said had crossed into its airspace, prompting speculation that the blast was a retaliatory move. Cars carrying refugees and trucks were lining up to cross into Turkey when the bomb exploded near a checkpoint on the Syrian side, witnesses said.

As international efforts to rid Syria of chemical weapons proceed, the government has stepped up its offensive against rebels, heightening concern that the diplomatic efforts could in fact intensify the conflict. President Bashar al-Assad’s forces claimed further gains in the Damascus suburbs Tuesday, taking the area of Shabaa near the airport road. Rebels denied those claims, but the pro-­Syrian Lebanese government TV channel al-Manar broadcast footage of its reporters touring what it said were rebel tunnels in the area.

The opposition forces have voiced frustration over the U.S. decision to hold back on military strikes against Syria and vowed to escalate their fight with expected new weapons shipments.

The conflict, now in its third year, has regularly overflowed into neighboring Turkey, Jordan and Lebanon. Meanwhile, The United Nations’ Middle East envoy, Robert Serry, told the Security Council on Tuesday that clashes between the Syrian army and rebels near the Israeli-occupied Golan Heights risk drawing Israel into the war.

The rebels, however, are also increasingly consumed by internal infighting. Last week, the al-Qaeda-linked Islamic State of Iraq and the Levant launched an operation it dubbed “Cleansing Evil,” saying the goal was to rid the northwestern city of al-Bab of groups it says are collaborating with the government, including the Islamist rebel Farouq Brigades and al-Nasr battalion.

Because Tuesday’s blast took place near a Farouq checkpoint, there were also suspicions that the Islamic State of Iraq and the Levant could have carried it out, but no group asserted responsibility. Juma al-Qassim, who said he was 50 yards away at the time, dismissed that possibility.

“If it was Islamic State, they would have driven right up to the checkpoint to hit Farouq,” he said. “But this clearly targeted civilians.”

Footage he said he filmed for the United Arab Emirates-based Al Aan television network showed thick black smoke rising from the crossing point as his car sped toward the scene.

“Everybody was running,” he said, adding that about 15 cars had been caught up in the blast. “Women were crying, children. It was terrifying. There were a lot of civilians and refugees.

The Turkish news agency Dogan said that in addition to the seven dead, at least 20 people were injured.