Lawmakers are pressing for a U.S. military option to help end the Syrian civil war, a so-called "Plan B," as the Obama administration's last-ditch diplomatic effort with Russia flounders.

Frustration on Capitol Hill is mounting after a temporary ceasefire negotiated Sept. 9 between the U.S. and Russia crumbled just a week later. The Syrian regime — backed by Russian air forces — then began a new onslaught on the city of Aleppo that has left hundreds of civilians dead.

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“I do think we should be looking at what other options we might have to take action in Syria that would change the dynamic and force Russia to recognize that it’s important for them to reengage in a resolution in Syria,” Sen. Jeanne Shaheen Cynthia (Jeanne) Jeanne ShaheenSenate Democrats introduce bill to sanction Russians over Taliban bounties Trump-backed candidate wins NH GOP Senate primary to take on Shaheen Democratic senator urges Trump to respond to Russian aggression MORE (D-N.H.), who sits on the Senate Armed Services and Foreign Relations Committees, told The Hill on Tuesday.

The administration says the strategy remains to end the war through diplomatic means, but insists it is keeping military options on the table.

Here are four possible options the administration could be weighing.

No-fly zone

The U.S. military, along with other partners, could impose a no-fly zone over Syria, or a large portion of the country. That would mean no aircraft would be able to fly in the area without permission, or risk being shot down.

Enforcing a no-fly zone or safe zone would require a number of aircraft to monitor and patrol the skies, take out threats or violators, conduct refueling and search and rescue missions.

A no-fly zone could also mean preemptively taking out regime systems that could pose potential threats, such as surface-to-air missile systems.

“You could do it with four or you might need 40 [aircraft],” said Ret. Air Force Lt. Gen. Ralph Jodice, who served as the air component commander for the 2011 NATO operation over Libya. “It depends on the size, it depends on what the potential threat is."

Critics of establishing a no-fly zone say it is too resource intensive, and that it would take away from the fight against the Islamic State in Iraq and Syria.

They also say it could risk bringing the U.S. into war with Russia or Syria, if they were to violate the no-fly zone and spark a confrontation.

But supporters of the zone argue that Russia would not risk war with the U.S. And they say a no-fly zone is better than the current situation, where the Syrian regime and Russian forces are bombing indiscriminately.

Safe zones

The U.S. military could also impose a safe zone, which would be a designated area where civilians can take refuge from military threats. That could help alleviate the flood of Syrian refugees flowing into surrounding countries and Europe.

Ret. Army Gen. Jack Keane, board chairman at the Institute for the Study of War and a former Army vice chief of staff, recommends forming two separate safe zones for displaced Syrians near the Turkish and Jordanian borders in Syria.

The zones could be protected on the ground by an international coalition of forces from the region, and possibly some from NATO, as well as from the air by U.S. Patriot missile systems in Jordan and Turkey, he told The Hill. The safe zones in effect would be no-fly zones, he said.

David Petraeus, the former CIA director and former head of U.S. Central Command, on Wednesday said "it's not too late" for such no-fly or safe zones.

"You can do that. That is very, very straightforward. Very, very quickly. And you don't even have to enter their airspace. You could do it with cruise missiles, air-launched, sea-launched and others," he said on PBS’s “Charlie Rose.”

Critics of the idea argue that a safe zone would require a lot of intelligence, surveillance and reconnaissance assets to detect violations, and ground forces to protect the zone.

But Petraeus said the U.S. now has local forces on the ground who could monitor the safe zones.

Michele Flournoy, who is widely expected to be Defense secretary if Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE is elected president, also expressed support for the idea of “no-bomb zones” in an interview earlier this year.

Target Assad’s air force

Another potential option would be grounding the air force of Syrian leader Bashar Assad.

Ret. Air Force Lt. Gen. David Deptula, dean of the Mitchell Institute for Aerospace Studies, said this option would be the easiest, cheapest way to get Syria to stop bombing civilians.

“In a general sense, the simplest, least expensive and most direct way would be to take out [Syria’s] delivery capacity... and that can be done very, very quickly,” said Deptula, who commanded no-fly-zone operations over Iraq in the late 1990s and directed the air campaign over Afghanistan in 2001.

He said grounding the air force would take far fewer resources than implementing a no-fly zone, and could be done within 24 hours.

He said this option, however, could cross over into “acts of war” against Syria -- something the administration has wanted to avoid thus far.

The U.S. military has already identified a number of Syrian military targets to hit, when it prepared for strikes after the regime used chemical weapons against civilians in the town of Ghouta in 2013. President Obama ultimately called off those strikes.

New weapons for rebels

Another option could be providing anti-aircraft systems to the rebels fighting the regime. That could help them take down Russian and regime aircraft, particularly low flying military helicopters dropping barrel bombs.

Those weapons could include man-portable air-defense systems (MANPADS) — shoulder-fired anti-aircraft missiles, which the U.S. has currently kept from the rebels.

The U.S. could use the same process it’s using now to supply vetted groups of Syrian rebels under a covert program with tube-launched, optically tracked, wire-guided (TOW) anti-tank guided missiles.

That program “has had a positive impact,” Keane said.

The Obama administration has had concerns antiaircraft weapons would fall into terrorists’ hands and be smuggled into the U.S. or used against U.S. air assets.

However, supporters of supplying MANPADS say their use could be safeguarded, much like the TOW missile program.

Keane said the anti-aircraft weapons could be supplied by the U.S. as well as allies.

“It’s still a worthwhile endeavor to attempt to shift the momentum against the Assad regime in an attempt to force a political solution which is not possible now under the current stalemate," he said.

“It’s still a worthwhile endeavor to attempt to shift the stalemate that currently exists,” he said.

Reuters recently reported that the U.S.’s Gulf allies are considering providing the anti-regime rebels with MANPADS, with Senate Armed Services Committee Chairman John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) also expressing support.

“It’s about time,” he told The Hill on Wednesday. “Because the [Obama administration] is not going to do it.”