By Tom Perry
BEIRUT (Reuters) - Syria's government on Saturday ruled out any discussion of the fate of the presidency at peace talks this week, just as the lead opposition negotiator said a political transition could not even start unless Bashar al-Assad was no longer president.
The statements reflected the huge challenges facing diplomats as they prepare for talks to resume in Geneva on Monday, trying to build on a ceasefire deal that has reduced violence sharply since Feb. 27.
Foreign Minister Walid al-Moualem confirmed his government's participation but said the talks would fail if the opposition had "delusions that they will take power in Geneva that they failed to take in battle".
He also heaped criticism on U.N. envoy Staffan de Mistura for already presenting an agenda for the talks and for saying that a presidential election would take place in 18 months.
"The government delegation will reject any attempt to put this on the agenda," Moualem told a televised news conference.
"We will not talk to anyone who talks about the position of the presidency ... I advise them that if this is their thinking, they shouldn't come to the talks."
Within hours, opposition negotiator Mohamad Alloush, already in Geneva, had described Moualem's comments as worthless.
"We consider that the transitional period starts with the fall of Bashar al-Assad or his death," he told reporters. "There's no possibility to start this period with the presence of this regime or the head of this regime in the power."
Another negotiator, Monzer Makhous, said Moualem was "putting the nails in the coffin of Geneva".
The talks will coincide with next week's fifth anniversary of a war that has killed more than 250,000 people, created the world's worst refugee crisis, and allowed for the expansion of the Islamic State militant group.
They are part of the first diplomatic push since the Russian air force intervened in September to support Assad, tilting the war his way and helping Damascus reclaim significant areas in the west.
The ceasefire agreement, brokered by the United States and Russia, has been more widely respected than many expected, though fighting has continued on some important fronts, including near the Turkish border.
Alloush's High Negotiations Committee (HNC) has praised the agenda outlined by de Mistura focused on governance, a new constitution and elections.
The HNC wants to focus on a transitional governing body with full executive powers as outlined in a 2012 Geneva communique in an early bid to end the conflict.
A U.N. Security Council resolution approved in December called for the establishment of "credible, inclusive, and non-sectarian governance", a new constitution, and free and fair elections within 18 months.
Moualem indicated that a "national unity government" with opposition participation was the most on offer, an idea ruled out by the HNC.
He said the government delegation would be willing to discuss de Mistura's agenda and would travel to Geneva on Sunday, but would return to Damascus within 24 hours if the other side did not show up.
As far as the government was concerned, "political transition" meant a transition from the existing constitution to a new one, and from the existing government to a new one with participation from the other side, he added.
The diplomacy has been complicated by disputes over who should be invited to negotiate with the government.
The Kurdish PYD party, which holds sway over wide areas of northern Syria, has so far been excluded from the talks in line with the wishes of Turkey - which sees the PYD as an extension of the PKK rebels fighting for Kurdish autonomy inside its territory.
Moualem said the Syrian army and the Kurds were in "one trench" fighting Islamic State, apparently in reference to the YPG militia, the PYD's armed wing, which has been battling the jihadist group in northern Syria with support from U.S.-led air strikes.
But Moualem ruled out the idea of federalism, one of the ideas backed by the PYD and mentioned by a Russian minister as a possible model for Syria.
The Russian Defence Ministry said it had registered 10 ceasefire violations in the previous 24 hours, but the truce was largely being respected.
Rebels did, however, shoot down a Syrian government warplane over western Syria on Saturday, rebels and a military source said, although there were conflicting accounts on whether it had been brought down by a missile or anti-aircraft guns.
Rebels have previously shot down Syrian warplanes with anti-aircraft guns.
They have asked foreign backers to supply them with anti-aircraft missiles but say they have not received any, reflecting fears that they could fall into the hands of Islamic State.
(Additional reporting by Omar Fahmy in Cairo and Marina Depetris, Firas Makdesi, Yara Abi Nader and Tom Miles in Geneva; Writing by Tom Perry and Kevin Liffey; Editing by Alistair Bell)