Kofi Annan, the joint UN-Arab League envoy on Syria, has said the international community should unite behind his mission to speak with one powerful voice to end the country's bloody crisis.
"Let me say one thing, if we are going to succeed it is extremely important that we all accept there should be one process of mediation - the one that the UN and the Arab League have asked me to lead," Annan told reporters on Wednesday after meeting with UN Secretary-General Ban Ki-moon in New York.
Annan, a former UN secretary-general, said he planned to go to Damascus "fairly soon" after leaving New York for Cairo. But he warned that Syrian President Bashar al-Assad might not agree to meet with him.
Syria has said it needs more information on his mission's goals before it will let him in, and has refused to allow Valerie Amos, the UN's under-secretary-general for humanitarian affairs, to enter the country.
Asked what message he would take, Annan said: "The message is clear, which is that the killing and violence must stop, humanitarian agencies must be given access to do their work. But it is regrettable that it is not happening."
"I would plead with him [Assad] that he should engage, not only with me but with the process we are launching."
Ban called on Syria's government to stop using force against civilians and said there was no more urgent crisis facing the international community.
"We seek a cessation of violence that sets the stage for a peaceful solution - a solution that offers all the Syrian people, regardless of religion or ethnicity or political affiliation, a future of security, dignity and freedom,” said Ban.
The diplomatic move came as Syrian government troops started advancing on parts of Homs, the country's third largest city, after weeks of artillery bombardment.
The latest assault, which appeared to have started overnight after power was cut to most of the area, saw troops clashing with fighters when they tried to enter the opposition stronghold of Bab Amr.
Opposition activists said heavy fighting erupted at the al-Bassel football field on the outskirts of Bab Amr.
There were reports that elite troops of the Fourth Armoured Division, commanded by Assad's brother Maher, had deployed with tanks around Bab Amr.
The activists also said Syrian soldiers abandoned checkpoints in the northern part of Homs, in the neighbourhoods of Al-Khaldia and Bayada.
The troops then started a heavy shelling campaign, the activists said, adding that there was sporadic shelling of other opposition areas in and around Homs.
"Homs is now and today experiencing the worst crackdown from Assad battalions and every district and residential area in Homs is being surrounded," Hadi Abdullah, an activist in the city, told Al Jazeera.
Soldiers are reportedly also searching houses and other buildings for army deserters who have since joined the opposition Free Syria Army.
"It is obvious that they are launching a major offensive against the strongholds of the Free Syrian Army," said Al Jazeera's Rula Amin, reporting from neighbouring Lebanon.
"President Assad and the government have made it clear; he has said that he wants this to be a decisive month. They believe that in this month they can contain the opposition, they can crush the armed opposition and that's what they are trying to do."
China backs aid
Meanwhile, the Chinese foreign minister said his country would back international efforts to send humanitarian aid to Syria, after Western powers proposed a UN Security Council resolution authorising humanitarian aid.
The US has drafted an outline for the resolution, which demands access for humanitarian aid workers to besieged towns and an end to the violence there.
"The pressing task now is for all sides to cease violence in the Syrian conflict, and to launch as soon as possible inclusive political dialogue and together deliberate on a reform plan," Yang Jiechi told Nabil Elaraby, the head of the Arab League, during a phone call late on Tuesday.
In-depth coverage of escalating violence across Syria
Yang was quoted by the official Xinhua news agency as saying: "The international community should create conditions for this, and extend humanitarian aid to Syria."
It was not clear whether Yang's remarks meant China would consider the proposed Security Council resolution.
China has been widely condemned for its handling of the Syria crisis. Elaraby has previously said China lost diplomatic capital in the Arab world after it joined Russia in vetoing two previous Security Council resolutions.
In a related development, Venezuela has said it is ready to send more fuel to Syria if requested, after confirming that two shipments of diesel were sent last year.
"They asked us on two occasions for diesel shipments and on two occasions, we provided them. They each contained 300,000 barrels," Rafael Ramirez, the Venezuelan oil minister, told journalists on Wednesday.
"If they ask us again, we'll give them more," Ramirez said. "We have a wide range of agreements with Syria," he said, adding that Syria was "a country harassed by imperialism."