BOQAYA, Lebanon Syria deployed tanks in a border village on Thursday, witnesses said, ignoring growing pressure from Washington which called on President Bashar al-Assad to stop trying to crush popular unrest or step aside.
"The Syrian regime has chosen the path of murder and mass arrest," Obama said in a condemnation of Assad's military campaign to suppress a two-month wave of protests demanding greater freedom.
Rights groups say more than 700 civilians have been killed in the repression, which continued on Wednesday as troops backed by tanks deployed in a Syrian border village, clearly visible from adjacent Lebanon.
"The Syrian people have shown their courage in demanding a transition to democracy," President Barack Obama said in a speech on U.S. policy in the Middle East and North Africa.
"President Assad now has a choice: He can lead that transition or get out of the way."
Obama called on Assad to stop shooting protesters, allow peaceful protests, release political prisoners and start a "serious dialogue to advance to a democratic transition."
Syrian security forces have used tanks, gunfire and mass arrests to crack down on flashpoints in an attempt to stamp out a revolt against four decades of authoritarian rule by the Assad family. Bashar himself has ruled for 11 years.
On Thursday Syrian soldiers could be seen deploying along a stream in Arida, a Syrian village next to Lebanon's northern border, and entering homes. Lebanese soldiers fanned out on their side of the frontier.
Earlier, sporadic gunfire and shelling were heard from the village. Arida is near the mostly Sunni Muslim town of Tel Kelakh, where one rights activist says Syrian troops have killed at least 27 civilians since entering on Saturday.
A resident said that armored personnel carriers and dozens of buses filled with soldiers had begun pulling out of Tel Kelakh around noon on Thursday and were heading north.
"The army is largely gone. There are no security police on the streets, but they are still in and around their headquarters away from the center of Tel Kelakh and they are still occupying the main hospital," the witness said.
Western powers, fearing instability across the Middle East if Syria undergoes a dramatic upheaval, at first made only muted criticisms of Assad's actions, but then stepped up their condemnation and imposed sanctions on leading Syrian figures.
Washington's decision to target Assad personally raises the question of whether the West will seek his overthrow, raising the stakes in the conflict.
Damascus condemned the sanctions, saying they targeted the Syrian people and served Israel's interests. "The sanctions have not and will not affect Syria's independent will," an official source was quoted as saying on state television.
"Any act of aggression against Syria is an American contribution to Israeli aggression against Syria and Arabs."
Leading Syrian opposition figure Haitham al-Maleh said the sanctions meant "members of the regime are now under siege."
The U.S. Treasury Department said it would freeze any assets owned by Syrian officials that fell within U.S. jurisdiction, and bar U.S. individuals and companies from dealing with them.
The sanctions also applied to Syria's vice president, prime minister, interior and defense ministers, the head of military intelligence and director of the political security branch, but it was unclear which assets, if any, would be blocked.
An EU diplomat said the European Union was also likely to extend its sanctions on Syria next week to include Assad.
"ASSAD MUST BE HELD RESPONSIBLE"
The unrest in Syria began in March when protesters, inspired by uprisings in other parts of the Arab world, took to the streets calling for democracy and an end to corruption.
Assad made token gestures toward reforms, including lifting a decades-old emergency law. But the crackdown by troops, security forces and irregular Assad loyalists intensified.
In a possible indication of the crackdown's ferocity, villagers near the southern city of Deraa said earlier this week they had found separate graves containing up to 26 bodies, allegations denied by Syrian authorities.
Ammar Qurabi of the National Organization for Human Rights in Syria said Assad had met two relatives of people reported to have been buried in the graves.
Many activists and protesters now say it is too late for reforms and are calling for Assad's overthrow. But the 45-year old president has shown no sign of allowing Western pressure to deter him from suppressing popular dissent.
Syrian authorities blame most of the violence on armed groups backed by Islamists and outside powers who they say have killed more than 120 soldiers and police.
Switzerland said it would impose travel bans on 13 top Syrian officials -- but not Assad -- and freeze any of their assets held in Swiss banks, matching a decision by the EU.
Amnesty International welcomed Washington's move and called on Obama to impose an arms embargo.
"President al-Assad and those around him must be held individually criminally responsible before the ICC (International Criminal Court) or national courts of states exercising universal jurisdiction," said Amnesty's T. Kumar.
Syria has excluded most international journalists, making it hard to verify reports from activists and officials.