Syria's tolerated opposition has rejected an offer to enter into talks with the government of President Bashar al-Assad to find a solution to end the 21-month conflict.

The National Coordination Body for Democratic Change in Syria (NCB) made the announcement on Monday even as Prime Minister Wael al-Halaqi said his cabinet would meet soon to draw up a mechanism for the peace roadmap announced by Assad.

On Sunday, Assad outlined in his first speech in months his vision to end the conflict that the UN says has killed 60,000 people.

The president called for an end to the violence and dialogue with opposition figures he deemed acceptable to chart a future course.

"We will not take part in a national dialogue before violence stops," the head of the opposition group, Hassan Abdel Azim, told a news conference in Damascus, setting the first of several conditions for talks with the regime.

He also demanded that any dialogue be preceded by the release of prisoners, a guarantee to ensure humanitarian aid is delivered to areas hit by the violence and the publication of a statement on the fate of missing Syrians.

"Any negotiation - not just a national dialogue - must be held under the aegis of the UN-Arab League envoy," he said.

Meanwhile, UN Secretary-General Ban Ki-moon said on Monday that Assad's rejection of the most important elements in an international roadmap to end the country's civil war means Assad has no real plan to end Syrians' "terrible suffering".

"The secretary-general was disappointed that the speech ... does not contribute to a solution that could end the terrible suffering of the Syrian people," said UN spokesman Martin Nesirky.

Ban and UN-Arab League envoy Lakhdar Brahimi will continue to work for a political transition that leads to UN-organised elections, he said.

The roadmap calls for a political transition and the establishment of a transitional governing body.

No 'political solutions'

Hours after Assad addressed cheering loyalists at the Damascus Opera House on Sunday, fighting erupted near the road to the city's international airport, the Syrian Observatory for Human Rights said.

The anti-regime activists said artillery hit the district of Aqraba, 5km from the Opera House.

Fighting continued all night and into Monday around the capital, as well as in the northern provinces of Idlib and Aleppo, it said.

In central Syria, the towns of Taybet Imam and Halfaya were bombarded with aerial strikes and artillery, said Abu Faisal, an activist speaking over the internet from Taybet Imam.

Abdel Azim and fellow NCB member Raja al-Nasser insisted that time had run out for "political solutions".

"Assad had to make such proposals at the start of the conflict, in April or May 2011, but he refused and opted for the security solution," Abdel Aziz said.

Nasser branded the initiative revealed on Sunday by Assad as a "war speech".

The official SANA news agency quoted Assad's embattled prime minister as inviting government ministers to meet "in order to set up the necessary mechanisms for the national programme" outlined by the president.

Assad outlined a three-phase plan, including entrusting the government with chairing a national dialogue conference with Syrians inside and outside the country to find a solution.

The NCB, which was set up in September 2011, six months after the outbreak of the Syrian conflict, brings together Arab and Kurdish nationalists, Marxists and independents, such as writer Michel Kilo and economist Aref Dalila.