Rebels have captured Syria's biggest hydro-electric dam and battled army tank units near the centre of Damascus, activists said as the opposition renewed an offer on Monday to negotiate the departure of President Bashar al-Assad.
On the Turkish border, nine people were killed when a car arriving from rebel-held territory in northwestern Syria blew up at the Reyhanli frontier crossing; Turkish officials said it was unclear whether the blast was a suicide attack or an accident.
The rebel seizure of the Taqba dam, a prestige project on the Euphrates river completed by Assad's father in the 1970s, may have only limited impact on already patchy power supplies but along with the fighting in the capital it provided more evidence that the president is ever more beleaguered, if still tenacious.
Moaz Alkhatib, exile leader of the opposition Syrian National Coalition, said he had had no response to his offer to discuss a handover of power with the Assad government but said the invitation still stood, despite the passing of an initial deadline for a response on Sunday.
The president has given no sign of wanting to negotiate his own departure, despite military reverses over the past two years that have put half the country in rebel hands and left many in his Alawite minority fearing for the worst if the mainly Sunni Muslim revolt overturns their four decades of pre-eminence.
On Monday, after Alkhatib's latest remarks on talks, Syria's state news agency quoted Assad as saying: "Syria will not give up on its principles however great the pressure and the conspiracies, which do not target Syria alone but all Arabs."
While Assad, an ally of Shi'ite, non-Arab Iran, has accused Sunni Arab states including Saudi Arabia and Qatar of fomenting the rebellion, he has also blamed Western powers and Israel of conspiring to overthrow him.
The British-based Syrian Observatory for Human Rights and other opposition activists said Islamist fighters seized army positions on Sunday around the Taqba dam, near the town of Thawra, some 150 km (90 miles) southeast of Aleppo. It was unclear how far the electricity plant was still operating.
"The dam was protected by an artillery battery and many intelligence units. The rebels moved on them in a lightning offensive yesterday, overrunning their positions and capturing scores of personnel," said Abu Ziad Teif, an opposition activist in contact with rebels in the area.
Rami Abdulrahman of the Observatory called it one of Assad's biggest setbacks. However, the dam's generating capacity has long fallen short of early hopes and, with power already scarce in Aleppo, Syria's biggest city, it was unclear whether the change of control at Taqba would have a major technical impact.
The loss of the neighbouring town, which Internet video showed included the burning of a statue of Assad's father Hafez was nonetheless symbolic. The large body of water formed by the dam on the Euphrates was named by the late leader Al-Assad Lake.
In Damascus, Assad's forces brought up tanks to defend an area just east of the city centre on Monday, residents said.
Jobar, a Sunni district adjacent to the landmark Abbasid Square, has seen fighting in recent days. However, activists said Assad's forces remained well dug in in the city centre.
"The main battle is taking place in Jobar," an opposition activist in Damascus named Amer said. "The rebels appear to be advancing in the eastern sector.
"But the centre of Damascus is crisscrossed with concrete barriers and security is deployed everywhere; we cannot say that they (the rebels) have a real active presence in the centre."
A woman who lives in the western neighbourhood of Mezze added that residents there were also beginning to feel the war getting near: "The situation is getting very tough," she said. "For the first time we've been hearing mortars fall so close."
In the north, on the Turkish border, officials said 33 people had been wounded as well as the seven killed near the town of Reyhanli: "We don't know whether this was a suicide bomb or whether a car that was smuggling petrol across the border blew up," one Turkish official told Reuters.
Images from the scene showed severe damage to cars at the border checkpoint, where a gate was blown open and part of the roof collapsed. "There was an explosion in the no-man's zone. It was not a mortar attack. It was very strong," foreign ministry spokesman Levent Gumrukcu said.
Refugees cross back and forth and Turkish trucks also deliver goods across into the no-man's land between the checkpoints of the two countries. Rebels control the Syrian side and receive help from Turkey, a fierce critic of Assad.
In Cairo, the opposition Coalition's Alkhatib met the head of the Arab League and said Assad's government had not responded to his initiative to discuss a transition of power.
"The regime has not given a clear answer so far, clearly, frankly, that it accepts leaving to spare destruction and blood," he told reporters. "No meetings have been arranged, and no formal contact with any party has happened so far."
Pressed to say whether his offer was still open despite the Sunday deadline passing, he added: "We are still waiting for the government response and then we are going to study that."
Also in Cairo, Assad's former prime minister Riad Hijab met the Egyptian foreign minister. Quoted by Egyptian news agency MENA, Hijab, the most senior government defector from Damascus, said: "There is no solution to the Syrian crisis except by the departure of Bashar al-Assad."
Hijab also told reporters the coalition was seeking Syria's national seat at the Arab League. A League source said the group would consult its other members on the request. (Additional reporting by Yasmine Saleh, Tom Perry, Alexander Dziadosz and Paul Taylor in Cairo, Jonathon Burch, Ozge Ozbilgin, Daren Butler and Nick Tattersall in Ankara; Writing by Alastair Macdonald; Editing by Giles Elgood)