The fighting forced dozens of wounded to pour across the volatile boundary to seek help on the Lebanese side, officials and activists said.
The fighting on the Syrian side was so intense that stray bullets and rockets landed on the Lebanese side, in the region of Wadi Khaled, according to Lebanon's state-run National News Agency.
A Lebanese man in Wadi Khaled was wounded and rushed to a nearby hospital, the report said.
The NNA said Syrian government forces closed the Bqaiaa crossing because of the fighting earlier on Thursday. Some stray bullets hit a Lebanese army checkpoint and two homes caught fire and burned in Lebanon, the report also said.
The Britain-based Syrian Observatory for Human Rights said the fighting concentrated in and around the Syrian village of al-Hosn, where 12 people were killed on Thursday.
The Observatory's chief, Rami Abdurrahman, said it was not clear if those killed were civilians or rebel fighters.
Syria's news agency SANA said "a number of terrorists were killed" as they tried to flee from al-Hosn toward Lebanon. Syrian authorities refer to opposition fighters as terrorists.
George Kittaneh, head of Lebanese Red Cross operations, told Lebanon's LBC television that about 45 wounded people were brought across the border into Lebanon for treatment.
Syrian troops backed by fighters from Lebanon's Hezbollah group have been on the offensive over the past months in areas close to the Lebanese border, aiming to cut rebel supply lines across the porous boundary.
Another Wadi Khaled resident, Ahmad al-Fahel, told The Associated Press over the telephone that schools were closed in areas close to the border and that some residents fled their homes to safer areas.
"People are terrified and many stayed in their homes," said al-Fahel, who owns a tanker truck that supplies residents with water from a spring near the border. "I haven't worked for two days."
Residents in some parts of northern Lebanon closed roads in protest of the shelling from Syria, NNA said.
Lebanon has been on edge since the central Syrian town of Yabroud was taken by President Bashar Assad's troops last Sunday.
Sectarian violence between Lebanon's Sunnis and Shiites linked to Syria's civil war has left scores of people dead in recent months.
In a spill-over from the conflict next door, pro- and anti-Syrian gunmen clashed overnight in the northern Libyan city of Tripoli. Fighting there has been ongoing since last week, with occasional lulls.
Also Thursday, Syrian state TV reported that troops discovered a makeshift rebel bomb-making factory used to rig cars with explosives in the border village of Ras Al-Ayn.
The village was captured on Wednesday by Syrian forces and Syrian state TV footage from the area showed what it said were vehicles with Lebanese plates that were meant to be sent to Lebanon for attacks.
About a dozen car bombs have gone off over the past months in Lebanon, killing dozens of people.
Hezbollah, the Shi'ite militant group in Lebanon that has been fighting on Assad's side in Syria's civil war, has said the vehicles were rigged with explosives inside Syria and subsequently sent to Lebanon.
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