The United States accused the Lebanese militant group Hezbollah on Friday of deep involvement in the Syrian government’s violent campaign to crush the uprising there, asserting that Hezbollah has trained and advised government forces inside Syria and has helped to expel opposition fighters from areas within the country.The American accusations, which were contained in coordinated announcements by the Treasury and State Departments announcing new sanctions against Syria, also accused Hezbollah of assisting operatives of Iran’s Revolutionary Guards Quds Force in training Syrian forces inside Syria. A Treasury statement said the Hezbollah secretary general, Hassan Nasrallah, had overseen those activities, which it called part of the Syria government’s “increasingly ruthless efforts to fight against the opposition.”

The accusations, which went beyond previous American charges about Hezbollah support for Syria’s government, seemed intended to counter critics of the Obama administration who say that the White House is not doing enough to support the Syrian opposition now that diplomatic efforts to resolve the conflict are paralyzed.

Some Hezbollah experts expressed considerable skepticism, however, saying that the accusations should be approached with caution unless more evidence was presented.

The accusations were also part of an effort to further draw attention to the Hezbollah-Iran alliance, which American and Israeli intelligence officials have sought to portray as a subversive collaboration that has not only destabilized the Middle East but has been implicated in terrorist violence elsewhere, including a deadly bus bombing of Israeli tourists in Bulgaria last month.