The Obama administration charged Friday that Lebanon-based Hezbollah is directly assisting Syria’s brutal domestic military crackdown and announced new sanctions against the group and the government of President Bashar al-Assad.

The announcements were timed to precede Secretary of State Hillary Rodham Clinton’s meetings on Saturday with Syrian opposition leaders in Turkey. Clinton also plans to pledge an additional $5.5 million in U.S. humanitarian aid for Syrian refugees.

The new measures are unlikely to placate Syrian rebel forces who have asked for U.S. military equipment and aerial protection in their increasingly bloody fight against Assad. Nor are they likely to please critics at home who say that President Obama is sitting on the sidelines of a humanitarian crisis and a battle that threatens U.S. security interests in the region, current and former U.S. officials said.

In sessions with Assad opponents and Turkish government leaders, Clinton plans to discuss options that do not go as far as direct U.S. intervention. The one-day stop follows her 10-day trip to Africa.

“She certainly will be looking to see whether there is anything else we can do that will have a positive impact rather than a detrimental impact on the overall situation in Syria,” a senior State Department official said Friday.

The cautious U.S. policy could change, as it did last year in Libya, despite the administration’s concern that adding arms to the volatile and increasingly sectarian civil war in Syria would only make things worse.

Clinton is looking for a “clear picture of the effectiveness of what we are currently providing and how it can be made more effective, and then whether or not there are additional things we can do,” said the official, who spoke on the condition of anonymity to discuss plans for the meetings.

But skepticism about the utility of any military assistance, a lack of international consensus and the upcoming U.S. presidential election make the possibility of any near-term military operation appear remote.

“I just don’t see it coming that fast, with or without the election,” another senior U.S. official said earlier this week, although that official and others agreed that domestic politics have complicated the response to the Syria crisis.

Amid fears that extremists are gaining strength within rebel ranks, some administration officials have argued internally that increased U.S. involvement would improve the likelihood of a democratic outcome and provide greater U.S. influence with the government that eventually replaces Assad. Republican presidential candidate Mitt Romney has said the United States should work with allies who are already supplying weapons to the rebels.

Armed with some tanks and heavy weapons supplied by Persian Gulf states or captured from the Assad army, the rebels have made significant gains in recent days, although not enough to shift the military balance of the 17-month conflict. On Friday, rebel forces acknowledged losing ground in the face of heavy government bombardment in the city of Aleppo but said they were preparing a counterattack, Reuters reported.

Residents were reported to be streaming out of the city into suburbs and a newly opened corridor into Turkey that is said to be controlled by the opposition but still under government artillery and air attack.

Obama has not ruled out any options in Syria, but White House counterterrorism adviser John O. Brennan said this week that opposition forces there are already “awash in weaponry.” There is little indication that a war-weary American public favors intervention in Syria.

The United States holds no uniform view of Assad’s staying power, with estimates ranging up to many months if he retains enough loyalty in his armed forces. But the pace of defections from his government and the growing military ability of the rebels have hastened the need for planning to head off a chaotic collapse of basic services and to prevent a security vacuum, officials said.

The administration is trying to expand its contact with political opposition figures beyond expatriate groups that have so far failed to mobilize support in Syria. Officials would not provide details about Clinton’s planned meetings in Istanbul but said the meetings would include activists who recently fled Syria or who travel in and out of the country.

The new U.S. sanctions on Hezbollah are likely to have more symbolic than substantive effect. The Shiite militia, whose political wing dominates the government in neighboring Lebanon, has long been supported by Iran and Syria. The Treasury Department first designated Hezbollah a “Global Terrorist” group in 1995, prohibiting U.S. financial transactions with it and freezing its assets.

Treasury Undersecretary David Cohen said the new action, focused on activities specifically related to Syria, was “not solely focused on the immediate financial impact” but was designed “to expose” Hezbollah activity in that country.

“As the wave of revolt has spread across the Middle East,” Cohen said, “Hezbollah leadership has publicly supported some protests where it suited their needs, and in other cases, such as in Syria, it has actively supported the violent crackdown being carried out by the Syrian dictatorship.”

Cohen and Daniel Benjamin, the State Department’s counterterrorism coordinator, declined to provide specifics about long-rumored Hezbollah participation in Assad’s military operation but said it included “extensive” logistical and operational support, as well as training and advice.

“This is not a matter of idle speculation or press reports,” Benjamin said.

“We’re obviously very sensitive here to issues of sources and methods,” he said, but added, “It’s safe to say that Hezbollah is playing a critical role in advising the Syrian government and its personnel in how to prosecute a counterinsurgency.”

Benjamin said that information had been compiled in an “authoritative document” distributed to other governments in hopes that they will take similar action against Hezbollah.

Separately, the administration imposed new sanctions on Syria’s state-run oil company, Sytrol, under the Iran Sanctions Act, which sharply limits energy-sector trade.

According to Treasury, Syrian shipments in April of 33,000 metric tons of gasoline to Iran — worth $36 million — benefited both countries, with Iran paying Syria with diesel fuel, used in military vehicles.

Since Syrian government entities are already subject to U.S. sanctions, the action primarily serves the broad purpose of drawing attention “to the really serious and deep relationships between the Iranian and Syrian regimes and the support that the Iranian regime has been providing to the Syrian government,” a senior administration official said.

Asked what the effect of the new actions would be, White House spokesman Jay Carney said, “The desired result is to continue to press the Assad regime, to continue to isolate it, and to expose the fact that Assad’s friends — and he has very few — are Iran and Hezbollah.”

Gearan reported from Accra, Ghana.