Syrian opposition figures have called for the overthrow of President Bashar Assad at a rare meeting of anti-government groups held in the government-controlled capital Damascus.

Rebels fighting Assad typically dismiss the so-called "internal opposition" as too lenient on the Syrian dictator, so the strong statements from the 16 parties in the National Coordination Body for Democratic Change in Syria may be aimed at gaining credibility among Syrians who despise the regime but are weary of an uprising that has since devolved into a bloody civil war. Assad's government tightly restricts criticism in areas it controls.

But the group may find it difficult to gain traction on its peace initiative, which is centred on a cease-fire. Many rebels look suspiciously at any political plan short of Assad's immediate ouster, seeing it as a play for time.

Ambassadors from Iran and Russia attended Sunday's conference. Both countries support Assad, suggesting the Syrian government authorized the gathering to bolster its position that there should be a peaceful settlement to the Syrian crisis through dialogue.

A statement distributed to journalists said the participants at the conference have agreed on a number of principles, mainly "overthrowing the regime with all its symbols" while emphasizing the need for "peaceful struggle to achieve the goals of the revolution."

"It's our right to meet here in the capital to express our views without being subject to dictates and pressures or to be forced to make concessions," the NCB's head, Hassan Abdul-Azim, said.

The Syrian opposition suffers deep divisions between the largely exiled opposition and those based inside the country. While agreeing on the need to topple Assad, the two differ on the means.

Unlike the Free Syrian Army rebel group and Syrian National Council made up largely of Syrian exiles, the NCB is opposed to the militarization of the Syrian uprising and any foreign military intervention. It is also more inclined to negotiate a peaceful settlement to the Syrian 18-month-old crisis.

NCB leaders, most of them traditional, leftist opposition figures, accuse the rebels and the SNC of being beholden to Turkey, which shelters defected Syrian generals and opposition figures, as well as Gulf Arab countries who support the rebels.

The rebels in turn accuse the NCB of being cut off from grassroots opposition fighters on the ground.

The statement that emerged from Sunday's conference called for an immediate ceasefire accompanied with the full withdrawal of the Syrian army from towns and cities and the release of all political detainees and kidnapped people. This would be followed by the start of negotiations between the opposition and representatives of the Syrian government on a peaceful transition of power, it added.

The scenario outlined by the participants is similar to a six-point peace plan proposed by the former international peace envoy to Syria, Kofi Annan. That plan ended with Annan quitting the post last month after his cease-fire failed to take hold as violence escalated across the country.

Missing NCB leaders

Syria's bloody 18-month conflict, which activists say has killed nearly 30,000 people, has so far eluded all attempts at international mediation.

Rajaa al-Nasser, a member of the NCB, said Damascus authorities have permitted all Syrian political figures to attend the conference "without restrictions."

However, the NCB has said two of its senior leaders disappeared after returning to Damascus International Airport from a trip to China on Thursday, along with a friend who was to pick them up. It has blamed the government for the disappearance.

The state-run news agency SANA quoted the Interior Ministry as saying "terrorist groups" kidnapped the three.