Russia and the United States' differences on the Syrian conflict were laid bare on Sunday as Lakhdar Brahimi headed to Cairo for talks with Arab leaders on his first visit to the region as peace envoy.
Troops meanwhile bombarded rebels in the key battleground city of Aleppo, as a bomb attack on a bus carrying civilians and soldiers in the flashpoint province of Homs reportedly killed four people and wounded dozens.
Brahimi, who last week described the Syrian bloodshed as "staggering" and the destruction "catastrophic", was on his way to the Middle East for the first time since taking up his role as the UN-Arab League envoy.
The veteran trouble-shooter took over from Kofi Annan, the former UN chief who quit the post in early August complaining of "continuous finger-pointing and name-calling" at the UN Security Council.
US Secretary of State Hillary Clinton said on Sunday that a new Security Council resolution on Syria would be pointless if it had "no teeth", as President Bashar al-Assad would ignore it.
Speaking in Russia, Clinton said she was willing to work with Moscow on a new resolution but warned Washington would step up support to end Assad's regime if the measure did not carry consequences.
"There is no point to passing a resolution with no teeth because we've seen time and time again that Assad will ignore it and keep attacking his own people," said Clinton.
Russian Foreign Minister Sergei Lavrov said on Saturday after talks with Clinton that he hoped to seek Security Council approval for a peace plan agreed in June in Geneva that called for a ceasefire and political transition.
Clinton said she hoped for progress but was "realistic" that the United States and Russia had differences on Syria.
If those differences persist, "then we will work with like-minded states to support a Syrian opposition to hasten the day when Assad falls," she said.
The United States has said it is providing non-lethal assistance to the opposition in Syria, whose regime which has been a Moscow ally since the Cold War.
Brahimi was travelling to the Egyptian capital on Sunday for talks with Arab League leaders ahead of a trip he hopes to make to Damascus, said his spokesman.
According to UN diplomats, Brahimi has been seeking guarantees that he will get a proper meeting with Assad before he goes to Damascus.
The Algerian former foreign minister said before taking up his post he was not confident of being able to restore peace, warning it was a matter of ending rather than avoiding a civil war.
On the ground, troops bombarded a central neighbourhood of Aleppo after a day of fierce clashes with rebels, the Syrian Observatory for Human Rights said.
Several houses were destroyed in the shelling of the northern city's Midan district, which is mostly under regime control, said the Britain-based watchdog.
Fierce battles broke out on the edge of Midan on Saturday, after rebels tried to take over the district from their stronghold in Bustan al-Basha.
A main water pipe in Bustan al-Basha was destroyed, either by air strikes or the fighting, and water shortages were reported by residents in Aleppo.
"The bombardment of Bustan al-Basha has stopped but could start up again anytime," a resident told AFP.
Aleppo's provincial governor blamed "terrorists" for breaking the watermain in Midan as well as two other pipelines in the Suleiman al-Halabi district.
"Maintenance crews are in the process of repairing them," Mohammed Akkad told pro-regime daily Al-Watan.
The Observatory reported an unknown number of casualties in Hanano after fierce shelling hit a building in the eastern neighbourhood.
On Saturday, troops backed by tanks and helicopters repelled a rebel attack on the Hanano military base after a 20-hour battle, according to state media.
In the central province of Homs, a bomb attack on a bus carrying civilians and soldiers killed at least four people and wounded dozens on Sunday, reports said.
"Four people were killed and others wounded in an explosion by a bomb which was planted by a terrorist group in a bus on the route from Homs to Messyaf," said state television.
The Observatory also reported the bombing, saying there were two explosions.
"We know for sure that four people were killed but we don't know if they were civilians or military," the Observatory's Rami Abdel Rahman told AFP.
"It was a large bus and there are dozens of casualties," he said, adding "the bombs were planted on the road".
In the capital, troops bombarded the southeastern district of Tadamun and nearby Al-Hajar Al-Aswad as clashes broke out at the Yarmuk Palestinian camp, the Observatory said.
Al-Watan said the army was "scouring" Tadamun after entering the district on Saturday in pursuit of "armed men".
Near the capital, clashes killed three rebels in Harasta, to the northeast of Damascus.
Overall, 19 people including a least 12 civilians had been killed nationwide so far on Sunday, the Observatory said.
The violence followed a bloody day in which 135 people -- 63 civilians, 33 rebels and 39 soldiers -- were killed nationwide, according to the watchdog's figures.