It was not clear if a major government offensive against Homs was under way, though shelling and gunfire resonated through the area, the U.N. Supervision Mission in Syria said in a written statement.
"Observers reported heavy fighting in Rastan and Talbiseh, north of the city, with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms," the statement said, "In Talbiseh, U.N. observers reported that the Free Syrian Army captured Syrian Army soldiers. The impact from heavy artillery shelling and machine gun firing was heard and seen over the neighborhood of Khaldiyeh in the city center."
Casualty reports were sketchy and unconfirmed. A large number of civilians were believed trapped in the combat zone. The United Nations said it was attempting to negotiate an arrangement that would allow the evacuation of civilians.
Opposition activists said at least 21 people were killed across Syria Monday, CNN reported.
Meanwhile, the new leader of the opposition Syrian National Council called on government officials to defect.
Speaking in Istanbul, Turkey, Abdulbaset Sieda said the group was not seeking "a foreign war or intervention. Rather it is the regime that is pushing our country in this direction; that is, waging unjust war on the nation and the people."
Sieda also called on officials in Syria, Russia and China "to think carefully about the situation now because the whole stability of the region -- if not the whole stability of the world -- is at stake here."
"We would like to call upon them to support the Syrian people," he said.
Russia and China, key trade allies of Syria, have blocked U.N. Security Council resolutions that many other nations said could have pushed Syrian President Bashar Assad to stop the killing. The two countries said they want more balanced resolutions that call on all sides to end the violence.
British Foreign Secretary William Hague said Western airstrikes in Syria remained a possibility. "Syria is ... on the edge of a collapse or of a sectarian civil war, so I don't think we can rule anything out," Hague said.
"But it is not so much like Libya last year, where of course we had a successful intervention to save lives," he told Britain's Sky News. "It is looking more like Bosnia in the 1990s, being on the edge of a sectarian conflict in which neighboring villages are attacking and killing each other."