Damascus vowed to keep fighting "terrorism" and the opposition insisted President Bashar al-Assad must go, as both sides dug in after a date for long-planned peace talks was set.
The Geneva peace conference scheduled for January 22 is aimed at ending the nearly three-year-old civil war, a bloody stalemate which has killed an estimated 120,000 people and driven millions from their homes.
But the opposition National Coalition, an umbrella group increasingly at odds with rebels on the ground, has insisted Assad have no role in the country's future, a demand long rejected by Damascus, casting doubt on whether a middle ground can be found.
The fighting has meanwhile shown no signs of abating, with a car bomb killing 15 people at a bus stop west of Damascus on Tuesday and battles heating up on key fronts east and north of the capital.
Iran -- a key ally of the Syrian regime which penned a landmark nuclear agreement with world powers on Sunday -- said Tuesday it was ready to take part in the so-called Geneva II conference, but would not accept any preconditions.
A day after the United Nations announced the date of the talks, the Coalition affirmed its "absolute rejection of Assad or any of the criminals responsible for killing the Syrian people playing any role in a transitional body... or in Syria's political future".
But it said it considers it "very positive" that a date has been set.
The opposition also called on world powers to "ensure humanitarian supplies reach all areas of Syria, while all prisoners must be set free" and that there should be "an immediate end" to massacres.
Rebel chief Selim Idriss said the Free Syrian Army would be ready to go to the talks if "the demands of revolutionaries on the ground", including Assad's fall, are met.
But while both the coalition and the FSA enjoy Western support, it's unclear how much control they have over the hundreds of rebel groups fighting on the ground, which include increasingly powerful jihadists battling both the regime and other rebel groups.
A newly-formed rebel alliance said Tuesday it wants to replace Syria's regime with an "Islamic state".
The covenant of the Islamic Front, Syria's largest armed opposition grouping with tens of thousands of fighters battling to oust Assad, spelt out its intention to play a role in politics and society as well as on the battlefield.
Under the subheading "democracies and parliaments," the Islamic Front said representative government "is based on the notion that the people have the right through institutions to (determine) legislation, whereas in Islam God is the sovereign".
The document added that the only way to bring about its objectives was through "military rebellion".
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The regime has meanwhile rejected any preconditions for peace talks, and Assad has said he would be willing to run for re-election in 2014.
Fighting rages on
Pro-regime daily Al-Watan on Tuesday quoted a letter from Foreign Minister Walid Muallem to the United Nations as saying "the war against the terrorism that is targeting Syrian citizens is key to ensuring the success of any peaceful solution".
He called on other countries to stop funding and sending weapons to the "armed terrorist groups", singling out Qatar, Saudi Arabia and Turkey by name.
Later on Tuesday, Iranian Foreign Minister Mohammad Javad Sarif said Tehran's "presence at the Geneva II conference will be important for resolving the Syrian crisis and we are ready to participate in the negotiations without any preconditions".
He was referring to a US demand that Iran first sign on to the Geneva I plan of June 2012, which envisaged the creation of a transitional government, but did not stipulate that Assad step down.
Fighting rages on
The fighting on the ground meanwhile appears likely to escalate as both sides seek to approach the negotiating table from a position of strength.
The suicide car bombing on Tuesday struck near a bus stop in Somariyeh, west of the capital, killing at least 15 people and wounding more than 30, according to state television.
Rebels, including Al-Qaeda loyalists, meanwhile pressed an offensive to lift a year-long army siege of eastern suburbs of Damascus.
Eight rebel commanders and 11 Lebanese Hezbollah militiamen are among scores of fighters killed on both sides in the past four days, said the Observatory.
Fighting also raged in the Qalamoun mountains north of Damascus, a border area where the army has been battling to cut rebel supply lines to neighbouring Lebanon.
Health Minister Saad al-Nayef accused the rebels of killing "five doctors, five nurses and two ambulance drivers" in the Qalamoun town of Deir Attiyeh after they seized it from the army last week.
In Nabuk, another rebel-held town in the region, government air strikes killed at least seven people, including three children, the British-based Syrian Observatory for Human Rights said.