Syrian rebels claimed gains in northern areas along the border with Turkey, while government troops pounded opposition strongholds in Syria's southern and central provinces, DPA reported.
As Syria and Turkey shelled areas on each side of their common border, the Syrian Observatory for Human Rights, an opposition group, said the rebels now control a large number of towns in the north.
In the latest developments, rebels captured two checkpoints from government forces in the province of Aleppo, rebel commander Abu Omar al-Halabi told dpa by phone. Taking control of Aleppo itself, Syria's largest city and economic hub, would represent a major boost for the rebels.
The Syrian and Turkish armies, meanwhile, continued to trade cross-border fire after a shell fired from Syria last week killed five Turkish civilians.
The governor of Hatay province in Turkey said a total of six Syrian shells had hit the Turkish side of the border, causing no casualties.
"All of them landed in rural areas," said Celalettin Lekesiz, in a statement carried by Anatolia, the state news agency.
Syrian state-controlled television said Syrian troops had advanced for the first time after four days of heavy shelling inside the Khalidiyeh region, in the central province of Homs.
The opposition London-based Syrian Observatory for Human Rights confirmed the report and said Syrian troops managed to occupy some buildings in the outskirts of Khalidiyeh.
On the diplomatic front, UN Secretary General Ban Ki-Moon made a strong appeal to stop the flow of arms into Syria and described the escalation in tensions along the Turkish-Syrian border as "extremely dangerous."
The situation within Syria had also "dramatically worsened" and was causing "serious risks to the stability of Syria's neighbours and the entire region," Ban's said in the French city of Strasbourg.
Violence across Syria killed at least 160 people on Monday, with 28 of them having died when the army launched an assault on the southern town of Karak, the Observatory said.
Syrian television said government troops also killed rebel commander, Abdul-Karim al-Shami, during shelling that targeted the area of al-Kussair.
"The sound of the shelling and black smoke on al-Kussair could be heard and seen from Lebanon," a resident of Wadi Khaled, in northern Lebanon, told dpa.
More than 31,000 people have been killed in the 19-month uprising against President Bashar al-Assad, according to opposition groups.
The United Nations High Commissioner for refugees (UNCHR) said in a new report that the number of Syrian refugees in Lebanon has now reached more than 85,200.
"Nearly 60,089 are currently registered at the UNHCR in addition to 25,150 refugees who have contacted the organization to be registered," the statement said.
An estimate 1,000 people crossed into the northeastern border town of Wadi Khaled from Damascus, Homs and Aleppo last week, and the UN agency now expects the tally of Syrian refugees in Lebanon to leap to 120,000 by the end of the year.
On the political front, the leader of Syria's main opposition group, the Syrian National Council, said its members planned to discuss in Qatar the possibility of Vice President Farouk al-Sharaa serving as interim leader should al-Assad step down.
"We will discuss the issue with other members of the opposition," Abdel Basset Sayda told the Dubai based Al Arabiya television.
Turkish Foreign Minister Ahmet Davutoglu said Saturday that al-Sharaa was a figure "whose hands are not contaminated in blood and therefore acceptable to Syrian opposition groups."