A blast killed at least 12 people Monday at a crossing point on the Turkish-Syrian border that has become a key supply line for rebel-held territory in northern Syria.

The explosion occurred amid fresh reports of rebel gains in several parts of Syria. It was unclear, however, whether the blast was deliberate, and Turkish officials said they are investigating the cause and circumstances of the explosion, which coincided with heightened Turkish concerns that the violence raging in Syria could spill over the border.

Turkish Deputy Prime Minister Bulent Arinc told reporters in Ankara that a van with Syrian license plates being driven into Turkey from Syria exploded just before the Turkish checkpoint at the Cilvegozu border crossing near Reyhanli, in Turkey’s Hatay province.

“But whether this was a vehicle laden with explosives or another type of explosion, I think, at the latest, will become clear tomorrow,” Arinc said. Four of the 12 victims were Turkish, and eight were Syrian; 28 people were injured, according to Turkish news reports.

The crossing, called Bab al-Hawa on the Syrian side of the border, has become a key transit point for supplies into the vast areas of northern Syria that are under rebel control. It also is a conduit for Syrians fleeing the relentless violence in their country. At least some of the blast victims were civilians seeking refuge in Turkey, activists said.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

Meanwhile, Syrian rebels announced that they had seized control of the country’s biggest dam, in Raqqah province, the latest in a series of gains in the remote northeast. The Furat dam on the Euphrates River provides electricity to many parts of Syria, and it was unclear to what extent the rebel seizure would affect power supplies.

According to the Britain-based Syrian Observatory for Human Rights, the radical al-Nusra Front led the assault on the dam, in the latest example of the growing role being played by the organization. Al-Nusra, also known as Jabhat al-Nusra, is suspected of ties to the al-Qaeda franchise. Videos posted online by activists showed the dam seemingly abandoned, and the Observatory said government forces had evacuated senior officials from the facility before it was overrun by the rebels.

Rebel fighters also said that they had seized control of a portion of the Damascus suburb of Zamalka as part of an intensified push to close in on the heavily fortified capital. After weeks of fierce battles in the suburbs ringing Damascus, the fighting has inched closer to the city center in recent days.

But the rebels still appear to lack the firepower to threaten President Bashar al-Assad’s tight control over the capital. At a meeting with Jordanian lawmakers Monday, Assad appeared as defiant as ever, telling them that Syria would remain “the beating heart of Arabism.”

“It will not relinquish its principles no matter how much pressure is exerted upon it and regardless of the conspiracies that not only target Syria, but also all Arabs,” Assad said, according to the official Syrian Arab News Agency.

His comment suggested that his regime still sees no need to reach out to the opposition, despite a bold offer by the head of the main opposition group for direct talks. Mouaz al-Khatib said Monday that he had received no response to his proposal for negotiations.

But the offer, made without consultation on Khatib’s Facebook page, has deeply divided his Syrian Opposition Coalition, with few members expressing support for it.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.