Rebels in Yemen seized one of the government's last strongholds in the country Thursday, leading diplomatic and military experts to speculate about how fighting may evolve on the oil-rich Arabian Peninsula.
Houthi rebels took central parts of the southern city of Aden, including the presidential palace, overnight. Meanwhile, officials said al-Qaeda militants in the country had freed about 300 inmates in the city of al-Mukalla, which experts say could boost the clout of extremist groups in the region.
A coalition of states led by Saudi Arabia has continued to pound rebel strongholds from the air as Yemen slips further into chaos, prompting worries that a more forceful intervention could become necessary. Yemen forms most of the southern border of Saudi Arabia, a top energy producer which holds about 17 percent of the world's proven oil reserves.
"If I were advising the Saudis, (I'd say) an air war is not going to do it for you, and a full ground invasion from the north is a very bad idea," said Nabeel Khoury, a non-resident senior fellow at the Atlantic Council, and the State Department's deputy chief of mission in Yemen in 2004-07. "You can insert a small force of elite soldiers, and put them in the south where they can intercept the Houthis if they further advance."
Khoury, who spoke with CNBC this week, said he would recommend employing that "carrot and stick" strategy to force the Houthis to come to the negotiating table.
The Saudi ambassador to the United States said on Thursday that his country does not have any "formal" ground force in Aden, but said such a move is open to consideration.
"The issue of using ground troops is always something that is on the table," Ambassador Adel al-Jubeir said at an event in Washington.
Hours after the Houthis took over Aden's central Crater neighborhood, unidentified armed men reportedly arrived by sea in an area of the port city which the Iran-allied fighters had not yet reached. A Yemeni official denied that ground troops had landed in Aden, and a port official said they were armed guards who had disembarked from a Chinese ship trying to bring aid or evacuate civilians.
Yemeni Foreign Minister Reyad Yassin Abdulla said that he could not confirm that coalition forces had landed in Aden, but told Reuters: "I hope so. I hope very much."
A Houthi spokesman said late on Wednesday that the fighting in Aden showed that Saudi Arabia's military intervention had failed.
"The victories in Aden today embarrass this campaign and silenced the aggressor states," Mohammad Abdulsalam told the group's al-Maseera television.
A diplomat in Riyadh said Aden had come to symbolize the fading authority of Yemeni President Abd-Rabbu Mansour Hadi and that Saudi Arabia could not afford to allow it to fall completely under Houthi control. But he said Riyadh's air campaign was so far geared more toward a slow war of attrition than an effective defense of the southern city.
That language was mirrored by U.S.-based experts, who told CNBC that the conflict would likely continue for some time.
"Right now the odds are we're looking at a longish war of attrition," Khoury said.
Paul Floyd, senior military analyst for global intelligence and advisory firm Stratfor, told CNBC last week that a ground offensive would be difficult near the rebel strongholds in the north of the country. However, truce negotiations would stand a chance if the Saudi-led air campaign could force a stalemate, he said.
For their part, the Saudis understand that a ground offensive would not be easy in Yemen, experts said.
"They know how difficult it is to pacify this country," Kamran Bokhari, author of "Political Islam in the Age of Democratization," told CNBC last week.
—Reuters and the Associated Press contributed to this report.