(Adds details on Assad forces advance, reports of rebels shooting down aircraft)
BEIRUT, Feb 12 (Reuters) - President Bashar al-Assad's forces and fighters from Lebanese ally Hezbollah stepped up attacks on Syria's strategic border town of Yabroud on Wednesday, activists said, in apparent preparation for a new offensive to flush out rebels.
The assault is the latest step in Assad and Hezbollah's campaign to secure the Lebanese-Syrian border region and fortify the president's hold on central Syria, from the capital Damascus to his stronghold on the coast.
Pro-government media said the army seized the nearby village al-Jarajeer, while the pro-opposition Syrian Observatory for Human Rights said Assad's forces had advanced on the area but had not completely taken it.
More than 13 air strikes had already hit the mountain border town on Wednesday, the British-based Syrian Observatory for Human Rights said, as overnight clashes between Assad's forces and the opposition on the outskirts of Yabroud continued into the morning.
"The attacks were sharply intensified but it is unclear if the offensive on Yabroud has started or if this is paving the way for a main attack," said Observatory head Rami Abdelrahman.
Arab news channel Al Meyadeen described the attacks as the start of a wider military offensive. It said Assad's forces were positioning themselves on hilltops outside Yabroud in preparation for attack.
Some activist groups said Hezbollah was involved in the fighting.
The attack on Yabroud is part of what locals have called the "Battle for Qalamoun", the name of mountainous region along the frontier with Lebanon used by both the rebels and Assad's allies to smuggle in people and supplies.
Rebels said they shot down an aircraft in the fighting on Wednesday, but there were no videos yet to verify the reports.
Such border clashes risk fuelling sectarian tensions in Lebanon, where Sunni-Shi'ite divisions deepened by the conflict in Syria have already triggered instability.
Rebels fighting to end four decades of Assad family rule are led largely by Syria's Sunni Muslim majority and have strong support from Sunnis in neighbouring Lebanon. Many of the fighting units in Yabroud are Islamist, including some with links to al Qaeda.
Assad's forces have support from minorities, particularly his Alawite sect, an offshoot of Shi'ite Islam. Assad's campaign in central Syria gained a huge boost from the support of Shi'ite Hezbollah's experienced militants, who fought a war against Israel in 2006.
The violence in Syria has set off a wave of tit-for-tat car bombings in Lebanon by both sides, as well as sporadic street clashes.
Lebanese media said at least one body and 22 wounded around Yabroud were transported to hospitals in Lebanon.
A new influx of Syrian refugees was reported in some of Lebanon's border towns a day earlier. Some told the Lebanese newspaper the Daily Star that Syrian army forces warned residents over mosque loudspeakers to flee the area if they wanted to save their lives.
Previous successful military assaults have given Assad the advantage along the Lebanese border. It was once a critical foothold for the rebels, whose main strongholds are now in Syria's northern and eastern regions as well as along parts of the southern border.
Syria's nearly three-year conflict began as peaceful protests against four decades of Assad family rule but devolved into an armed conflict after a security force crackdown. (Reporting by Erika Solomon; Editing by Andrew Heavens)