Syrian security forces are advancing on the rebel-held neighbourhood of Baba Amr in the city of Homs, an official says.
The area has been heavily shelled by tanks, artillery and rockets for weeks already by security forces trying to quell the 11-month-old uprising to oust President Bashar Assad.
But the statement today indicated a ground offensive was under way.
Activists say hundreds of people have been killed in the area since February 4, when the assault on Homs began. Activists and residents inside Baba Amr could not be reached early today.
The official vowed Baba Amr would be "cleaned" within hours.
The statement came hours after British photographer Paul Conroy was smuggled out of the city across the border into neighbouring Lebanon.
Thirteen Syrian activists who were helping were reported killed in the operation.
Three foreign journalists are thought to be still in the city.
Two other Western journalists - American Marie Colvin and French photographer Remi Ochlik - were killed there last week.
Meanwhile the French government confirmed today that two French reporters remained trapped in Baba Amr, Edith Bouvier of Le Figaro and William Daniels. Bouvier and Conroy were wounded last week in the same attack that killed American Marie Colvin and Remi Ochlik.
The French Foreign Ministry demanded that the Syrian regime ensure conditions that allow for the "sure and rapid evacuation" of the two French journalists, "notably through an immediate ceasefire in Baba Amr."
"France is mobilised to accomplish the priority evacuation of its two citizens blocked in Homs, in liaison with Syrian authorities," as well as the Red Cross and Red Crescent, it said.
Spanish reporter Javier Espinosa, who works for the major Spanish daily El Mundo, also remained trapped in Baba Amr, El Mundo said.
The paper said it had spoken briefly to him on Tuesday evening and he said he was "ok." The paper said it did not think Espinosa is injured and did not know where he is staying.
Troops also entered the central town of Halfaya in the province of Hama, near Homs, after five days of intense shelling, activists said. They said the rebel-held town of Rastan, just north of Homs, was subjected to shelling and casualties were reported.
On Tuesday, the UN raised its death toll for the almost year-long uprising to 7,500, an increase of 2,000 from just two months earlier. Syrian activists say the toll has surpassed 8,000.
The UN High Commissioner for Human Rights Navi Pillay has cited a UN expert panel's report that concluded Syrian government officials were responsible for "crimes against humanity" committed by security forces against opposition members.
Syria's state-run news agency said three gunmen were killed and others were wounded as they tried to cross from Lebanon into Homs province. It said aSyrian soldier was wounded.
Also today, China urged world powers to provide humanitarian assistance to Syria, as Beijing tries to bolster diplomacy while continuing to oppose any armed outside intervention in the conflict.
A Chinese Foreign Ministry spokesman said that Foreign Minister Yang Jiechi broached the idea of assistance in telephone conversations with the head of the Arab League and the foreign ministers of Saudi Arabia and Algeria.
China defied Western powers and the Arab League by joining Russia to veto a UN resolution that outlined plans to end the conflict and condemned Assad's crackdown on anti-government forces.