U.S. Secretary of State Hillary Rodham Clinton, left, meets with Ghana's President John Dramani Mahama, at his residence in Accra, Ghana, on Aug. 9, 2012. On Saturday, Clinton will make her way to Istanbul for meetins on Syria’s enduring conflict. (Jacquelyn Martin/AP)

The Obama administration is unlikely to broaden military engagement in Syria before the U.S. presidential election, despite rebel military gains, pleas for help from the rebels and criticism at home that President Obama is sitting on the sidelines, current and former U.S. officials said.

The officials agree that the gradual expansion of U.S. support for the Syrian rebels will stop well short of any armed intervention or aerial protection zone for now.

The United States imposed more economic sanctions on Syria on Friday and will announce an additional $5.5 million in humanitarian aid for Syrian refugees Saturday officials said. The Treasury Department also announced a new terrorist designation of Hezbollah, in neighboring Lebanon, saying the group is providing “a range of critical support” for Syrian President Bashar al-Assad’s government, including “training advice, and logistical and operational” assistance.

Secretary of State Hillary Rodham Clinton plans to discuss other options Saturday, during emergency meetings in Istanbul with Turkish government leaders and opponents of Assad. The one-day stop in Turkey follows a 10-day diplomatic 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 U.S. calculus of caution could change, as it did last year in Libya, despite the administration’s current policy 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,” the official said.

But a combination of skepticism in the United States about the utility of any military move, a lack of international consensus and domestic political worries makes the possibility of any near-term military operation appear remote.

The upcoming U.S. presidential election in November casts the national security decision-making on Syria in a political light. Obama administration officials insist they are neither postponing nor hastening any policy change because of the election, but officials agree that unless Assad falls quickly, the United States is highly unlikely to significantly alter its current course before then.

“I just don’t see it coming that fast, with or without the election,” one senior U.S. official said earlier this week. The official, like others, agreed that the election does complicate the already difficult effort to understand the changing situation in Syria and react to it.

There is a debate within the administration about what to do next, with some advisers arguing that some wider help for the rebels would give the United States greater influence with the government that eventually replaces Assad, and would improve the chances for a democratic outcome.

Obama administration officials bristle at criticism from Republican presidential candidate Mitt Romney and others that the United States has been a bystander and should arm the rebels. Doing so might provoke a wider war, with little gain for the United States, two senior U.S. officials said this week.

John O. Brennan, the White House’s top counterterrorism official, said Wednesday that Obama has not ruled out any options for helping the Syrian rebels, although he noted that they already are “awash in weaponry.”

American public opinion has solidly favored winding down the Afghan war and the war in Iraq before it, and the public mostly sides against any new military intervention in Syria. There have been few calls, even from foreign policy hawks, for anything on the scale of the U.S. invasion of Iraq in 2003.

The administration is expanding contact with political opposition figures who might be leaders in Syria after the Assad regime falls, and it has gradually ratcheted up the level of assistance to the splintered military resistance inside Syria. It is now providing satellite equipment and sophisticated radios that allow the rebels to better coordinate their movements and detect regime attack helicopters and other heavy weaponry.

Clinton has never met any of the activists she will see Saturday, two State Department officials said. She will meet no armed fighters or commanders, they said. Previous meetings with opposition groups have revolved around an umbrella group of political exiles.

Armed with some tanks and heavy weapons supplied by Persian Gulf states or captured from the Assad army, the rebels have made significant gains, although not enough to shift the military balance of the 17-month conflict.

At the same time, a peace plan put forward earlier this year by U.N. envoy Kofi Annan has collapsed.

The plan, which included a cease-fire that never took hold, was not taken seriously even by some of its most ardent public backers, because they assumed that Assad would never go along. However, the plan did serve to answer the question of what the United States was doing to help. It also could have given cover to Russia, Syria’s close partner, to negotiate a political deal for Assad to step down.

The United States and several allies are likely to shortly endorse a replacement for Annan, who quit after the plan collapsed, and United Nations monitors are likely to maintain a small, and largely bunkered, presence in the country, officials said.

The changed circumstances are putting pressure on the United States, Turkey and European allies to seize the opportunity and help the rebels, perhaps with more weapons or some form of military protection from the air.

U.S. officials appear no closer to that kind of intervention, however. Clinton has led a gradual embrace of the opposition forces over the past half-year that now includes provision of sophisticated communications and other “nonlethal” military gear. Significant expansion of the U.S. role is unlikely in the short term, and there is little appetite in Turkey for a strong military response, despite worry over the consequences of a prolonged civil war at its doorstep.

Other U.S. officials said a goal of the Istanbul trip is to ensure that Clinton sees a more diverse array of opposition figures than the longtime expatriates she has met. Although U.S. officials did not provide names or significant detail about the possible participants, some are likely to be activists who recently fled Syria or who travel in and out.

The officials spoke on condition of anonymity because the list of participants is not set, and they cautioned that identifying some of some activists publicly would put them in greater danger.

The Treasury’s move to designate Hezbollah a terrorist organization follows decades in which the United States has prohibited financial transactions with the group and frozen its assets. Treasury Undersecretary David Cohen said the action was “not solely focused on the immediate financial impact” but was designed “to expose” Hezbollah activity in Syria. Assad is a longtime supporter of Hezbollah, along with Iran.

U.S. officials declined to provide specifics about long-rumored Hezbollah operational support for Assad’s crackdown during the Syrian uprising that began early last year, but said that intelligence had now provided significant evidence.

“This is not a matter of idle speculation or press reports,” said Daniel Benjamin, the State Department’s counterterrorism coordinator. “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 they will take similar action against Hezbollah.

The United States holds no uniform view of Assad’s staying power, with estimates ranging to many months if he retains enough loyalty in his armed forces. Rebel retreat from part of Aleppo under heavy air assault over the past few days shows that the Assad regime is still in control, military and other officials said Friday. Syrian forces have pushed rebels back from a strategic district of the country’s commercial hub, although skirmishes continue in the city.

But Clinton’s stepped-up engagement this week is a recognition that the end is coming, and perhaps much sooner. The pace of defections and the growing military ability of the rebels hasten the need for planning to head off a chaotic collapse of basic government services and to prevent a security vacuum in Syria once Assad goes, officials said.

That is what Clinton meant when she appealed earlier this week for thoughtful consideration of the “day after” the fall. She said she “couldn’t possibly predict” when that day will come.

The rebels also say they do no want direct military intervention in the form of troops on the ground. But they have repeatedly appealed for a no-fly zone similar to the effort that helped Libyan rebels topple Moammar Gaddafi last year and for supplies of heavy weapons to counter Assad’s vastly superior firepower.

The Washington Post reported this week that as the Arab world’s bloodiest revolt continues, anti-American sentiments are hardening among those struggling to overthrow Assad.

Once regarded by the Syrian opposition as a natural friend in its struggle for greater freedoms against a regime long at odds with the West, the United States is now often being viewed with resentment for offering little more than verbal encouragement to the revolutionaries.

“All we get is words,” said Yasser Abu Ali, a spokesman for one of the rebel Free Syrian Army battalions in the town of al-Bab, 30 miles northeast of Aleppo.

The violence already carries signs of sectarian conflict between Syria’s majority Sunni Muslim community and Assad’s minority Alawite sect, an offshoot of Shiite Islam.

“There will be no winner in Syria,” U.N. Secretary General Ban Ki-moon said in a statement read by a U.N. representative Thursday. “Now, we face the grim possibility of long-term civil war destroying Syria's rich tapestry of interwoven communities.”

Karen DeYoung in Washington contributed to this report.