After welcoming hundreds of thousands of Syrian refugees in the early years of the conflict, Lebanon banned them from entry in 2015. AFP
Millions of Syrians have fled abroad since their country's civil war broke out in 2011 following the government's repression of peaceful pro-democracy protests.
Many have crossed the border into Lebanon, which the United Nations says hosts the largest number of refugees per capita in the world.
The Lebanese army said in a statement that it had "prevented around 1,200 Syrians from crossing the Lebanese-Syrian border in the past week".
It had announced on August 23 that it turned back 700 Syrians attempting to enter the eastern Mediterranean country irregularly.
Prime Minister Najib Mikati on Thursday expressed concern about a "new wave" of refugees crossing the border "via illegal paths".
"The army and the police are working to prevent" this, he added.
Lebanon, which has been mired in a crippling economic crisis for more than three years, says it hosts nearly two million Syrians. The United Nation has registered almost 830,000 of them.
Anti-Syrian sentiment has soared in recent months as some officials have sought to blame refugees for the country's woes.
A security official told AFP that "the Syrian-Lebanese border is porous and the number of soldiers mobilised is not enough".
"Most Syrians come to Lebanon in the hope of finding work, given the unprecedented deterioration in living conditions in their country," said the official who spoke on condition of anonymity as they were not authorised to speak to the media.
Last month, the Damascus government scrapped fuel subsidies, dealing a further blow to Syrians reeling from 12 years of war and a crippling economic crisis.
The conflict has killed more than 500,000 people and ravaged the country's infrastructure and industry.
Most of the population has been pushed into poverty, according to the United Nations.
After welcoming hundreds of thousands of Syrian refugees in the early years of the conflict, Lebanon banned them from entry in 2015.
Since then, many Syrians have used smugglers to cross the border and seek other opportunities in Lebanon or beyond.
Lebanon's own economic collapse has also turned it into a launchpad for would-be migrants, with Lebanese joining Syrian and Palestinian refugees clamouring to leave via dangerous sea routes across the Mediterranean.