BEIRUT -- Thousands of Syrians poured into Lebanon, taking shelter in wedding halls and makeshift shacks after fleeing heavy fighting in a mountainous region across the border in Syria, while a massive explosion Sunday targeting a government building outside Damascus reportedly killed at least 31 soldiers.
The clashes in Qalamoun, an area that stretches north of the Syrian capital along the Lebanese frontier, appeared to be part of a long-anticipated government offensive aimed at cutting an important rebel supply route and cementing President Bashar Assad's hold on a key corridor from the capital to the coast.
A government victory in the strategic region would deal a severe blow to the already beleaguered rebels on Damascus' doorstep.
Over the past month, Assad's forces have made headway against the rebels on two key fronts, capturing a string of opposition-held suburbs south of Damascus and taking two towns and a military base outside the northern city of Aleppo.
Still, the opposition remains firmly entrenched in other areas around Damascus and capable of carrying out large attacks. A massive bombing Sunday leveled a government office in the northeastern suburb of Harasta, killing at least 31 soldiers, according to the Britain-based Syrian Observatory for Human Rights.
Three brigadiers and one major general were among the dead, according to the group's director, Rami Abdurrahman. There was no immediate confirmation from government officials or state media.
Since the heavy fighting in Qalamoun began Friday, some 10,000 Syrians have fled across the border to the Lebanese frontier town of Arsal, former Mayor Bassel Hojeiri said.
He said the new arrivals have crammed into wedding halls and improvised shacks.
Some families left so quickly that they arrived in Lebanon "without anything except the clothes on their backs," said Dana Sleiman, who works for the United Nations High Commissioner for Refugees.
She said at least 1,000 Syrian families crossed into Lebanon over the weekend, but many had not yet registered with the U.N., so more precise figures weren't available.
Ms.Sleiman said some of the new arrivals settled into tin shack slums that dot eastern Lebanon's Bekaa Valley, and they were being offered thick plastic to reinforce their shelters against the cold.
The U.N. refugee agency was also distributing blankets, mattresses, food, diapers and hygiene kits to the refugees.
The new refugees join an estimated 1.4 million Syrians -- 800,000 of whom have registered -- who have already found shelter in Lebanon, according to Lebanese officials.
The massive influx has become a burden for Lebanon, and has helped stoke the country's already simmering sectarian tensions.
Ms. Sleiman said most of the Syrians who crossed into Lebanon over the weekend were from the town of Qara, which is the focal point of the offensive along with the nearby towns of Rima and Nabak.
The battle for Qalamoun has been expected for weeks, and both the government and the opposition have been reinforcing their positions in the region ahead of winter, when much of the area is covered with snow.
Qara holds strategic value because of its location on the main highway leading from Damascus to the central city of Homs.
If government troops gain the upper hand, they will be able to cut supplies that flow from Lebanon to rebel-held areas around Damascus while also maintaining control over movement from the capital to the coast, which is a government stronghold.
On Sunday, two pro-rebel activist groups and the Observatory reported two airstrikes on Qara. They said the highway was severed and regime forces had gathered on nearby hills, trying to cut supplies to rebels inside the town.
Syria's pro-government media said the battle would be decisive.
Now in its third year, Syria's conflict has killed more than 120,000 people, according to the Observatory, which closely monitors the violence in Syria through a network of activists across the country.
The U.N. said in July that 100,000 Syrians have been killed, and has not updated that figure since. Millions of Syrians have been uprooted from their homes because of the fighting.
Meanwhile, a series of mortar rounds hitting the center of Damascus killed four people, the Syrian official news agency SANA said.
While mortar fire into the capital is becoming a regular occurrence, residents said the shelling from nearby rebel-held areas into the center was particularly heavy this week.
The capital and much of southern Syria also experienced a power outage Sunday evening, the state news agency said. Electricity Minister Imad Khamis blamed the blackout on a rebel attack.