Kofi Annan, the UN-Arab League envoy to Syria, said he would urge President Bashar al-Assad and his foes to stop fighting and seek a political solution, drawing angry rebukes from dissidents.
"The killing has to stop and we need to find a way of putting in the appropriate reforms and moving forward," Annan said on Thursday in Cairo ahead of his trip to Damascus on Saturday.
Syrian dissidents reacted with dismay and said government repression had destroyed prospects of a negotiated deal. More than 7,500 people have been killed in a year-long crackdown on an uprising against Assad, according to the United Nations.
"We reject any dialogue while tanks shell our towns, snipers shoot our women and children and many areas are cut off from the world by the regime without electricity, communications or water," said Hadi Abdullah, contacted in the city of Homs.
Another activist told Reuters Annan's call for dialogue sounded "like a wink at Bashar" that would only encourage Assad to "crush the revolution".
UN humanitarian chief Valerie Amos, on a separate mission to Syria, said she was "devastated" by the destruction she had seen in the Baba Amr district of Homs city, and wanted to know what had happened to its residents, who endured a 26-day military siege before rebel fighters withdrew a week ago.
Amos is the first senior foreign official to visit Baba Amr since the government assault.
As world pressure on Syria mounted, the deputy oil minister announced his defection, the first by a senior civilian official since the start of the uprising. Abdo Hussameldin, 58, said he knew his change of sides would bring persecution on his family.
Two rebel groups later said four more high-ranking military officers had defected over the past three days to a camp for Syrian army deserters in southern Turkey.
Lieutenant Khaled al-Hamoud, a spokesman for the Free Syrian Army (FSA), told Reuters by telephone the desertions brought to seven the number of brigadier generals who had defected.
In Damascus, the authorities continued to crack down on Assad opponents, with government forces shooting and wounding three mourners at a funeral for an army defector that turned into a protest against the president, locals said.
Opposition sources and residents say protests in the capital are driven by inflation and the plunging value of the Syrian pound.
The world has failed to stop an unequal struggle pitting mostly Sunni Muslim demonstrators and lightly armed rebels against the armoured might of Assad's 300,000-strong military, secret police and feared Alawite militiamen.
Western powers have shied away from Libya-style military intervention in Syria, which sits at the heart of a conflict-prone Middle East.
Defense Secretary Leon Panetta on Wednesday defended US caution about military involvement, especially without international consensus on Syria, but said the Pentagon had reviewed US military options.
Tunisia and Turkey, a neighbour of Syria, have also declared their opposition to intervention by any force from outside the region, and Annan argued against further militarisation of the conflict.
"We should not forget the possible impact of Syria on the region if there is any miscalculation," the former UN chief said.
But Syrian dissidents said diplomatic initiatives had proved fruitless in the past. "When they fail no action is taken against the regime and that's why the opposition has to arm itself against its executioner," said one rebel army officer.
Russia, a staunch defender of Syria, said Assad was battling al Qaeda-backed "terrorists" including at least 15,000 foreign fighters who it said would seize towns if Assad troops withdraw.
"The flow of all kind of terrorists from some neighbouring countries is always increasing," Russia's deputy ambassador Mikhail Lebedev said in Geneva.
The Libyan government denied Russian accusations that it was running camps to train and arm Syrian rebels.
On the ground, the humanitarian situation appeared dire. The United Nations said it was preparing food supplies for 1.5 million Syrians as part of a 90-day emergency plan.
"More needs to be done," John Ging of the UN Office for the Coordination of Humanitarian Affairs, which is headed by Amos, told a Syria Humanitarian Forum in Geneva.