There has been intense shelling by the Syrian army overnight in the city of Homs - described by opposition activists as the worst in weeks.
Eleven United Nations monitors are now stationed in Homs to try to implement a ceasefire there.
But a BBC correspondent in Homs says they have an impossible task, with both sides breaking the truce.
On Wednesday, a convoy carrying UN observers was hit by a roadside bomb blast in the southern city of Deraa.
The head of the UN team, Maj Gen Robert Mood, was in the convoy, but neither he nor any of the other monitors was hurt.
Eyewitnesses said at least three Syrian soldiers were wounded. The windows of the truck were shattered.
UN Secretary General Ban Ki-moon said such incidents could threaten the future of the UN mission in Syria.
The observers are in the country as part of the joint UN-Arab League peace plan and began deploying last month.
There are now about 70 monitors in Syria as a whole but their presence has had no effect in quelling the violence.
'Dance of death'
An Associated Press reporter travelling in the UN convoy said the explosion in Deraa blew out the military truck's windows and caused a plume of thick black smoke.
It is not clear who was behind the blast. However, the opposition Syrian National Council is blaming the government, saying the explosion is part of a campaign to drive the UN monitors out of the country.
Meanwhile, the BBC's Lyse Doucet says there is constant shooting in Homs, despite a ceasefire between government and opposition forces.
She saw UN observers patrolling the city but said entire neighbourhoods were deserted.
In other developments, UN Middle East envoy Terje Roed-Larsen told the Security Council that arms were being smuggled in both directions between Lebanon and Syria.
"What we see across the region is a dance of death at the brink of the abyss of war," he told reporters later, AFP reported.
Violence was reported in several parts of the country on Wednesday, including the northern province of Idlib and the city of Hama.
Also on Wednesday, Syrian troops fired across the border into Lebanon, killing an elderly woman and wounding her daughter, Lebanese officials said.
The UN estimates about 26,000 Syrians have fled across the border to Lebanon, most of them in the north.
The UN says at least 9,000 people have died since pro-democracy protests began in March 2011. In February, Syria's government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.
Syria heavily restricts access to foreign journalists and the reports cannot be independently verified.