National Coalition opposition chief, Ahmed Moaz al-Khatib, meanwhile, said he had received "no clear response" from Damascus over his offer of dialogue.
Khatib said in late January he was prepared to hold direct talks with regime representatives without "blood on their hands," on condition the talks focus on replacing Assad.
The Assad regime has said it was open to talks but without conditions attached.
Khatib, speaking to reporters in Cairo, proposed that direct talks with regime representatives could take place in "liberated areas" of rebel-dominated northern Syria.
On the border between northern Syria and southeast Turkey, at least 13 people were killed and dozens wounded when a car exploded, Turkish officials said, although the motive was not immediately clear.
"We have unfortunately lost 13 people: three of them Turks and rest Syrians," Prime Minister Recep Tayyip Erdogan said.
The powerful blast was caused by unidentified explosives and "all possibilities are on the table, including political motives," Deputy Prime Minister Bulent Arinc said after a cabinet meeting.
A foreign ministry official told AFP the likelihood of it being a terrorist attack was "51 percent."
The blast hit a busy parking lot barely 40 metres (yards) into the buffer zone from the Cilvegozu crossing in the Turkish town of Reyhanli near the Syrian border and triggered a fire that damaged around 15 vehicles, the ministry official said.
Another Turkish foreign ministry official said a suicide bomber might have been involved in the blast that smashed apart the gates at the crossing, opposite Syria's Bab al-Hawa post.
The blast coincided with the planned time of arrival of an opposition delegation at the frontier, a top official of the opposition Syrian National Council said.
"We had a meeting with the joint command of the (rebel) Free Syrian Army, and the explosion took place at the exact moment when we were supposed to arrive" at the crossing, Abdel Basset Sayda said, adding their arrival had been delayed.
Turkey, a one-time Syria ally which is now vehemently opposed to Assad's regime, has taken in almost 200,000 refugees from the conflict and serves as a transit point for rebel fighters.
-- Largest dam seized --
On the ground, rebels on Monday seized control of the largest dam in Syria, a vital barrier along the Euphrates River in the northern province of Raqa that generates 880 megawatts of power, the Syrian Observatory for Human Rights said.
"This is the biggest economic loss for the regime since the start of the revolution," which the United Nations says has cost more than 60,000 lives since it broke out in mid-March 2011, Observatory head Rami Abdel Rahman told AFP.
Rebels from the jihadist Al-Nusra Front and the Awayis al-Qurani and Ahrar al-Tabqa battalions met little resistance in the area, as loyalist security chiefs fled on board military helicopters, he said.
The capture of the dam is the latest in a string of key rebel victories in northern and eastern Syria but the insurgents have yet to take a major city in the war-ravaged country.
Elsewhere, warplanes bombarded two districts of southern Damascus, Assali and Qadam, said the Observatory.
Rebels in the northern city of Tabqa burned down a massive statue of late president Hafez al-Assad, Bashar's father and predecessor, according to amateur video distributed by the Observatory.
The Observatory, which relies on a vast network of activists on the ground and medics, said at least 111 people -- almost equally divided between civilians, government forces and rebels -- were killed in violence across Syria on Monday.