A top military commander and at least 18 other senior officers defected Monday to the opposition movement demanding the ouster of Yemen's embattled president, depriving the U.S.-allied ruler of most of his power base.
The looming collapse of President Ali Abdullah Saleh's regime throws into doubt the American campaign against a major al-Qaida wing that plotted attacks in the United States.
Monday's defections led to rival tanks being deployed in the streets of Yemen's capital, Sana, creating a potentially explosive situation and prompting Saleh's defense minister, Mohammed Nasser Ahmed, to announce the military remained loyal to the longtime leader.
The armed forces will counter any plots against the government, Ahmed declared on state television, following a meeting of the National Defense Council, which is led by Saleh.
The defection of Maj. Gen. Ali Mohsen al-Ahmar, a longtime Saleh confidante and commander of the army's powerful 1st Armored Division, was seen by many as a turning point. It followed a major escalation in the regime's crackdown on demonstrators.
Speaking in Paris, French Foreign Minister Alain Juppe called Saleh's resignation "unavoidable" and pledged "support to all those that fight for democracy."
Tanks, armored vehicles and soldiers directed by al-Ahmar fanned out around the Sana square that has become the epicenter of the opposition movement, moving in for the first time to protect demonstrators.
Al-Ahmar also sent tanks to the state television building, the Central Bank and the Defense Ministry.
The deployment of al-Ahmar's troops in Sana was greeted by wild jubilation from protesters, many of whom posed with soldiers for photographs, greeted them with military style salutes or offered them roses.
In a sign of the Obama administration's growing alarm over the regime's crackdown on demonstrators, State Department spokesman Mark Toner called on the Yemeni leader to refrain from violence.
"We abhor the violence. We want a cessation of all violence against demonstrators," Toner said, calling on Saleh to "take the necessary steps to promote a meaningful dialogue that addresses the concerns of his people."
The president, 65, most recently has battled a seven-year armed rebellion in the north, a secessionist movement in the south and an al-Qaida offshoot that is of great concern to the United States.
Al-Qaida in the Arabian Peninsula, formed in 2009, has moved beyond regional aims and attacked the West, including sending a suicide bomber who tried to down a Detroit-bound airliner on Christmas Day with a bomb sewn into his underwear. The device failed to detonate properly.
Yemen is also home to U.S.-born radical cleric Anwar al-Awlaki, who is believed to have offered inspiration to those attacking the United States, including Army Maj. Nidal Hasan, who is accused of killing 13 people and wounding dozens in a 2009 shootout at Fort Hood, Texas.