BENGHAZI, Libya – Forces loyal to Libyan dictator Moammar Gadhafi advanced farther east Sunday, moving into the coastal city of Brega, putting new pressure on the United States and its allies to either intervene militarily or risk seeing the anti-Gadhafi movement collapse.

For rebels battling Gadhafi in the nearly month-old uprising, if they cannot retake Brega, it would be the fourth city they have lost in three days, a dramatic reversal of fortune since they swept out of eastern Libya last month and seemed days away from the outskirts of the capital, Tripoli. Three of the four cities — Zawiya, Brega and Ras Lanuf — have refineries.

While discussions of an international no-fly zone have intensified since the Arab League’s endorsement Saturday, it remains unclear whether such a step could stop Gadhafi’s overwhelming firepower — or, even if it could, whether it would come in time.

On Sunday, rebel fighters said they moved to the outskirts of Brega after coming under attack by missiles fired from ships at port and from the air.

The developments appeared to set up a confrontation between rebels and pro-Gadhafi forces in the city of Ajdabiya, a major gateway to the rebel capital of Benghazi, Libya’s second-largest city.

Control of Ajdabiya would give Gadhafi control of a major crossing point, and, in theory, the prospect of surrounding Benghazi by moving some of his forces directly to Tobruk in far northeastern Libya.

President Obama has demanded that Gadhafi quit, but he has employed little leverage to accomplish that goal.

The 22-member Arab League, normally hostile to outside intervention in its region’s internal affairs, took the unprecedented step Saturday of asking the U.N. Security Council to impose a no-fly zone over Libya to prevent Gadhafi from using military aircraft.

The United States and other Western nations wanted that endorsement as political cover, but imposition of a no-fly zone seems days away at best. And it would not stop Gadhafi from using his ground forces to attack the rebels.

In Washington, a senior Obama administration official described the Arab League decision as “an important step,” but added that there “are still other hurdles to overcome.”

The official said it remained an open question whether Arab powers would take part in imposing a no-fly zone, a condition the Obama administration has said it wants to avoid the perception that the action would be unilateral U.S. intervention into a region where anti-U.S. sentiment is high. “As we’ve said, we want to see both regional endorsement and regional participation,” the official said.

Obama’s national security adviser, Thomas Donilon, said Thursday that the White House wants more than rhetorical support from Libya’s neighbors.

“We’re going to be seeking actual support by those nations — the Arab League, the (Saudi-led Gulf Cooperation Council) and the African nations — to participate in any of these efforts as they go forward,” Donilon said. “Again, not just rhetorical support, but actual participation.”

Secretary of State Hillary Rodham Clinton is due to discuss Libya with her counterparts at a meeting today of foreign ministers from the G-8 group of nations in Paris.

Sen. Charles Schumer, D-N.Y., said Sunday on NBC’s “Meet the Press” that the Arab League’s decision “makes a no-fly zone more likely.”

Instead of a no-fly zone, which the rebels know the United States is hesitant to enforce, the opposition forces said they want the West to provide more advanced weapons than those they took from looted military bases. Yet many rebels do not know how to use the weapons they have now.

The state of fighting in and around Brega was murky Sunday. Cell-phone access there and in nearby Ajadbiya appeared to be limited, if there was any, cutting off the rebels’ main means of communication.

On Friday, Gadhafi’s regime claimed to have recaptured Zawiya, 30 miles west of Tripoli, after days of siege, another major setback to the rebels.