Syria: Annan ceasefire holds despite violent incidents
Violence has fallen in Syria as a fragile ceasefire takes hold, although both sides are reporting violations.
Syrian state TV said a roadside bomb in the city of Aleppo had killed one person and wounded 24.
The opposition said three people had been killed in Idlib and Hama and there was "no evidence" of a significant withdrawal by Syrian troops.
US President Barack Obama and French President Nicolas Sarkozy called on Syria to respect the ceasefire.
In a statement issued by the White House, the two leaders said they would intensify efforts to stop the "brutal crackdown" in Syria, including at the UN Security Council.
The UN is now exploring the possibility of deploying monitors to Syria.
One diplomat at the UN in New York told the BBC the idea was to deploy a team of about 30 observers within the next 48 hours to test the waters for a full mission.
Negotiations on a draft resolution on a monitoring mission are to start later on Thursday, says the BBC's UN correspondent Barbara Plett, with the aim of getting it adopted on Friday.
Earlier, UN-Arab League envoy Kofi Annan, who brokered the ceasefire as part of a six-point peace plan, said he was "encouraged" but that Syria had not fully complied.
"All parties have obligations to implement fully the six-point plan. This includes both the military provisions of the plan and the commitment to move to a political process," Mr Annan told the UN Security Council.
UN Secretary General Ban Ki-moon said the onus was on the Syrian government to match words with deeds.
"The world is watching, however, with sceptical eyes, since many promises previously made by the government of Syria have not been kept," Mr Ban told a news conference in Geneva.
The ceasefire came into force early on Thursday.
Little violence was reported at first, but the UK-based Syrian Observatory for Human Rights later said one man had been shot dead at a checkpoint in Hama province.
At about 15:30 local time (12:30 GMT), Syrian state TV reported what it described as a roadside bomb in Aleppo which killed one military officer and wounded 24, said to be officers and cadets.
It said "terrorists" were escalating attacks to destabilise the country and sabotage the Annan plan.
An activist video posted online, which could not be verified, appeared to show a government tank and machine guns opening fire in Homs, a centre of anti-government protest, and a sniper in place on a rooftop.
Activists also accused the security forces of shooting at people staging a protest in Idlib, shelling areas of Hama and firing on a funeral at Aleppo in the north.
"There is no evidence of a significant withdrawal," opposition Syrian National Council (SNC) spokeswoman Basma Kodmani told a news briefing in Geneva.
The BBC's correspondent in Beirut, Jim Muir, says the level of violence has clearly dropped sharply, but that it is clear the ceasefire is far from perfect.
If the ceasefire does hold, the focus is expected to fall on the withdrawal of government troops, tanks and heavy weaponry, he says. That step was supposed to have been completed on Tuesday.
The truce will also be a test of how unified and disciplined the inexperienced rebel army can be, our correspondent adds.
'Freedom to protest'
Syrian Foreign Ministry spokesman Jihad Makdissi earlier told the BBC the government was "fully committed" to the cessation of all military operations "as long as we have no armed violence against the state".
"If those people at the external opposition want the full benefit of their own country, they have to come to the table of negotiation and table of dialogue."
Col Kassem Saadeddine, of the main armed rebel group, the Free Syrian Army (FSA), said that if the regime "does not stop shelling and not withdraw tanks, we will intensify our military operations and launch attacks".
Neighbouring Turkey said it would be "very happy" if the Annan plan succeeded.
Turkey is hosting more than 24,000 refugees from Syria. Incidents of cross-border shooting have injured at least six people on Turkish soil.
Foreign Minister Ahmet Davutoglu told the BBC that if the peace collapsed and the international community failed to respond, or if there were a "national security risk" to Turkey, it had "the right to make any preparation to protect the border and to help people who are escaping from massacres".
The UN estimates about 9,000 people have died since anti-government protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.