Syrian government forces have been continuing their assault with artillery, ground forces and helicopter gunships on rebel-held areas of the second city, Aleppo.
Officials said one area, Salah al-Din, had been recaptured, but rebels said the battle there was still going on.
UN observers have reported an upsurge in violence in the city, the new head of their mission Babacar Gaye said.
He added that he had personally seen heavy shelling in the city of Homs.
Lt-Gen Gaye also said he had witnessed serious damage from shelling and fighting in the nearby town of Rastan.
UN Secretary General Ban Ki-moon later reported that Gen Gaye's convoy had been attacked by government tanks at the weekend.
He said no-one was injured, as personnel were protected by the vehicles' armour.
In other developments:
Greece is to quadruple the number of guards on its border with Turkey to pre-empt a possible influx of Syrian refugees, AP quotes Public Order Minister Nikos Dendias as saying
The al-Jazeera media network says one of its journalists, Ankara-based correspondent Omar Khashram, has been wounded by shrapnel in Aleppo and evacuated to Turkey for treatment
Syria's most senior diplomat in London, Charge d'Affaires Khaled al-Ayoubi, says he has left his post and is no longer willing to represent a regime that has "committed such violent and oppressive acts against its own people", a UK Foreign Office statement says
A Turkish official tells Reuters news agency that 12 police officers, including the chief of Latakia city's force, have fled over the border
The Turkish army has sent reinforcements including missile batteries to the Syrian border, state-run Anatolian news agency reports
'Not one metre'
Government forces launched a ground assault on Aleppo on Saturday after a week of sporadic shelling and sorties by fighter jets.
With fighting into its third day, UN observers and witnesses reported fire from mortars, tanks and helicopter gunships.
Fighting has focused on the Salah al-Din neighbourhood in Aleppo's south-west, where the rebels had embedded themselves.
Syrian state television showed footage from the city and interviewed soldiers who said they had taken complete control of Salah al-Din late on Sunday.
On Monday, officials in Damascus again said they had "purged" the area.
But activists have denied that the quarter has been overrun by the army, saying rebels are still in control.
The head of the Free Syrian Army in Aleppo, Col Abdel Jabbar al-Oqaidi, told the AFP news agency the government "had not progressed one metre".
Heavy shelling and clashes have also been reported in the Sakhur quarter on the north-east side of the city centre, where another attack by government forces appeared to be under way.
And an AFP reporter said rebels had captured a checkpoint at Anadan, 5km (three miles) north-west of Aleppo, seizing government armoured vehicles.
Correspondents say that controlling the checkpoint would give the rebels a direct route between Aleppo and the Turkish border.
Meanwhile UN humanitarian chief Baroness Valerie Amos said that, according to Red Cross and Syrian Arab Red Crescent estimates, some 200,000 people had fled fighting in Aleppo.
She said others were trapped in the city and needed urgent help.
"I call on all parties to the fighting to ensure that they do not target civilians and that they allow humanitarian organisations safe access."
She said many people had fled their homes to take shelter in schools and other public buildings.
The BBC's Ian Pannell, in the Aleppo area, says residents are facing food shortages and power cuts.
He says the rebels are outgunned by the army, but they are fighting an effective guerrilla war in the streets.