Hezbollah -- whose paramilitary wing is widely seen as Lebanon's strongest military force -- has also sent fighters into Syria, Lebanese officials and analysts told The Washington Post.
U.S. officials have not confirmed the group's combat role. A senior U.S. intelligence official told the Post he believed the group, which Washington has classified as a terrorist organization, had stopped short of carrying out operations or attacks.
A Hezbollah spokesman declined to comment on allegations the single most powerful group in Lebanon was involved in the Syria conflict.
Three Lebanese officials who are members of a political bloc opposed to Hezbollah pointed to evidence Hezbollah fighters were battling Syria's opposition.
They told the Post dozens of "martyrs" were quietly being buried in Hezbollah areas of Lebanon. They said the families of the dead were warned not to discuss how their sons died.
Hezbollah militants killed in clashes with Israel typically receive large public funerals.
Obituaries for Hezbollah fighters have also started appearing in local newspapers without the cause of death explained, one of the officials told the Post.
Washington accused Hezbollah Aug. 10 of deep involvement in the Bashar Assad regime's violent campaign to crush the uprising, alleging the group, whose name means "Party of God," had trained and advised regime forces inside Syria and helped remove rebel fighters from parts of the country.
The accusations, in coordinated announcements by the U.S. Treasury and State departments, also accused Hezbollah of helping operatives of a special operations unit of Iran's Revolutionary Guards known as the Quds Force train Syrian forces inside Syria.
Washington considers the Quds Force a terrorist organization.
The Treasury Department followed up on the August accusations with new sanctions against Hezbollah's leadership Sept. 13, "further exposing Hezbollah's active support to the regime of Syrian President Bashar Assad, as well as its role in terrorist activities."
The department accused Hezbollah of "providing training, advice and extensive logistical support to the government of Syria."
Hezbollah fighters are expert in guerrilla warfare -- fighting the Israeli military to a standstill in 2006 -- and could complement the Syrian military, whose experience is largely limited to conventional warfare, the Post said.
Hezbollah is also helping the Assad regime with its news and propaganda, Lebanese political activist Lokman Slim told the Post.
Communication specialists from Hezbollah's al-Manar satellite TV station, which Washington calls a "specially designated global terrorist entity," are helping Syria's official TV channel present polished news reports from the regime's viewpoint, Slim said.
Slim runs Hayya Bina, a civic initiative that seeks to make Lebanese politics less sectarian and is "committed to resisting the culture of fear and intolerance," its website says. The group is a frequent critic of Hezbollah.
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