The U.S. accused the Lebanon-based Hezbollah militant political group of providing support for Syrian President Bashar al-Assad, including training and military advice to help the regime crack down on a popular uprising.

By going public with the allegations, the Obama administration is trying to put one of Mr. Assad´s few remaining allies in the region on notice that its activities are being monitored, U.S. officials said.

The U.S. said Hezbollah has been working closely with Iran, Mr. Assad´s other close supporter and the main U.S. nemesis in the region.

"I think it is safe to say that Hezbollah is playing a critical role in advising the Syrian government and training its personnel in how to prosecute a counterinsurgency," Daniel Benjamin, the State Department´s coordinator for counterterrorism, said Friday. "At this point…we´re satisfied by our assessment that the group is playing an absolutely integral role in helping the Assad regime try to put down this popular movement."

It was unclear how the U.S. announcement would affect Hezbollah, which has long been the target of U.S. sanctions for what Washington deems terrorist activities. U.S. officials said it is unclear whether the new allegations will lead to any additional penalties but said it could spur other countries to respond more forcefully to Hezbollah.

Syria has long provided support to Hezbollah, offering haven for training camps and safe passage of arms from Iran and elsewhere, according to U.S. and Israeli officials.

"Hezbollah is now repaying its debt to Assad," said Treasury Under Secretary for Terrorism and Financial Intelligence David Cohen.

Analysts in the region argue that the U.S. is overstating Hezbollah´s role in Syria, and say the group has chosen to limit its support for Mr. Assad—endorsing the regime but not publicly taking up arms to defend it.

"Although the final decision hasn´t been made, Hezbollah has come to realize that it doesn´t serve its core objectives to get involved in Syria and Iran´s conflict," said Omar Nashabi, a columnist for the Al- Akbar newspaper in Lebanon, which leans toward supporting Hezbollah.

There have been several incidents of violent clashes between Mr. Assad´s supporters and opponents in Lebanon, but Hezbollah avoided getting involved. When Syrian rebels kidnapped 11 Lebanese pilgrims, Hezbollah leader Hassan Nasrallah called for restraint.

Separately, the U.S. Embassy in Beirut said Friday it has "received reports of an increased possibility" that Americans visiting or residing in Lebanon may be attacked. "Possible threats include kidnapping, the potential for an upsurge in violence, the escalation of family or neighborhood disputes, as well as U.S. citizens being the target of terrorist attacks in Lebanon," the statement said. It didn´t identify which groups or individuals present the heightened threats.

U.S. officials said Hezbollah´s support for Mr. Assad´s crackdown began nearly 17 months ago, soon after antiregime protests broke out. Hezbollah´s role inside Syria has ramped up as the fighting has intensified, they said. Activists have said the uprising in Syria has claimed more than 19,000 lives.

According to the U.S., Hezbollah operatives have trained Syrian government personnel inside Syria and have facilitated training by the paramilitary arm of Iran´s Revolutionary Guard Corps, known as the Qods Force.

The U.S. accused Hezbollah, in addition to providing operational and logistical support to the Syrian government, of playing an active role in helping expel Syrian opposition forces from some areas of Syria. Officials provided few details.

The allegations about Hezbollah´s role in Syria come amid mounting U.S. concerns that Hezbollah and Iran have stepped up terrorist plotting around the world as tensions with the U.S. and Israel have grown over Iran´s nuclear program.

Also on Friday, the Obama administration imposed sanctions on the Syrian state-run oil company, Sytrol, accusing it of conducting transactions with Iran in violation of U.S. sanctions.

Mr. Benjamin said Hezbollah could carry out attacks in Europe and Asia, including Thailand, where a previous plot was foiled. U.S. officials said last month´s bombing in Bulgaria, which killed five Israelis, bore the hallmarks of Hezbollah but that a final determination about who was responsible for that attack has yet to be made.

"We assess that Hezbollah could attack in Europe or elsewhere at any time with little or no warning," Mr. Benjamin said. The U.S. hasn´t, however, detected any operational activity by Hezbollah inside the U.S., he added.

—Sarah Portlock and Sam Dagher contributed to this article. (Copyright © Dow Jones & Company, Inc.) 08/11/12)