Lebanon has told more than a million Syrian refugees they will lose their refugee status if they cross back into Syria, two days before a presidential election in which Syrian officials have urged many to vote at polling stations on the border.
Tens of thousands of Syrians cast ballots at their Beirut embassy in an early round of expatriate voting last week and embassy staff said those who were unable to participate could vote at polling stations just inside Syria on Tuesday.
The scale of turnout at the embassy and the vocal displays of support for President Bashar al-Assad angered Assad's Lebanese opponents, who said any refugees who took part should be sent back to Syria.
Lebanon's Interior Ministry made no mention of the election in its warning to the Syrian refugees, but said it was acting to "prevent any friction or mutual provocation'' between Syrians and their Lebanese host communities.
"All displaced Syrians and those registered with the U.N. High Commissioner for Refugees are asked to refrain from entering Syria as of June 1, 2014, under penalty of losing their refugee status in Lebanon,'' the ministry said.
The three-year-old conflict has deepened political divisions in Lebanon and fuelled violence including bombings, rocket attacks and gunbattles. The Hezbollah-dominated March 8 political coalition supports Assad while its March 14 opponents have backed the rebels trying to topple him.
The presence of a million Syrians escaping a seemingly endless conflict has also raised fears that Lebanon's 4 million people may end up hosting a permanent refugee population several times larger than the Palestinian influx which helped destabilize the country before its 1975-1990 civil war.
UNHCR spokesperson Dana Sleiman said the ministry's message had been relayed to the refugees, but that the agency had told the Lebanese government that refugees who return to Syria may still fear persecution or face serious danger.
"Some Syrian refugees return briefly to their home country, including to renew their documents, check on elderly or sick family members or property, and to see if the situation in their villages is safe enough for return,'' she said.
"Some refugees also face pressure or coercion to return and are not necessarily returning voluntarily. Some have been told if they do not return to vote they will never be re-admitted to Syria when conditions are conducive to safe and voluntary return.''
Assad is widely expected to win a third seven-year term in Tuesday's election, which is being held in the midst of a civil war which has killed 160,000 people, destroyed large parts of Syrian cities and driven nearly 3 million refugees abroad.
Large swathes of the country remain out of Assad's control although his forces, backed by Lebanon's Hezbollah militant group and Iraqi Shi'ite fighters, have forced the Syrian rebels and foreign jihadis to retreat in central Syria.
The conflict erupted after Assad, whose family has ruled Syria for four decades, used force to crush protesters who took to the streets to demand change in March 2011.