Yemen's U.S.-backed president, his support crumbling among political allies and the army, warned that the country could slide into a "bloody" civil war Tuesday as the opposition rejected his offer to step down by the end of the year.

Tens of thousands protested in the capital demanding his immediate ouster, emboldened by top military commanders who joined their cause.

Ali Abdullah Saleh's apparent determination to cling to power raised fears that Yemen could be pushed into even greater instability. In a potentially explosive split, rival factions of the military have deployed tanks in the capital Sanaa — with units commanded by Saleh's son protecting the president's palace, and units loyal to a top dissident commander protecting the protesters.

The defection on Monday of that commander, Maj.-Gen. Ali Mohsen al-Ahmar, a powerful regime insider who commands the army's 1st Armoured Division, has been seen by many as a major turning point toward a potentially rapid end for Saleh's nearly 32-year rule.

The question is whether the Yemeni chapter of the uprisings sweeping the Middle East will read more like Egypt — where the resignation of President Hosni Mubarak set the country on a relatively stable, if still uncertain, move toward democracy — or like Libya, which has seen brutal fighting between armed camps.

Clashes broke out late Monday between Saleh's Republican Guard and dissident army units in the far eastern corner of the country. On Tuesday, Republican Guard tanks surrounded a key air base in the western Red Sea coastal city of Hodeida after its commander — Col. Ahmed al-Sanhani, a member of Saleh's own clan —announced he was joining the opposition.

The turmoil raised alarm in Washington, which has heavily backed Saleh to wage a campaign against a major Yemen-based al-Qaeda wing that plotted attacks in the United States.

Al-Qaeda threat concerns U.S.

U.S. Defence Secretary Robert Gates, on a trip to Russia, said Tuesday that "instability and diversion of attention" from dealing with al-Qaeda is a "primary concern about the situation." He refused to weigh in on whether Saleh should step down.

After a month of street protests — led mainly by students and pro-democracy advocates — against his nearly 32-year rule, Saleh became dramatically more isolated after security forces opened fatally shot more than 40 demonstrators on Friday.

The killings set off an avalanche of defections by top figures in his ruling party, influential tribal leaders and, most damagingly, al-Ahmar and a string of other top generals.

In a meeting Tuesday with his still-loyalist military commanders, Saleh railed against the dissidents, calling them "weak" and saying they "dropped away like autumn leaves."

"Those who want to climb to power through a coup should know that things won't stabilize. The nation won't be stable, it will turn into a civil war, to a bloody war, so they should think carefully," he said.

At the same time, he issued a softer statement, saying he "sympathizes with the youth" and calling on protesters to enter a dialogue. He said he believed their movement could "renew the democratic energy" in the country."

Monday night, Saleh pledged in a meeting with senior officials, military commanders and tribal leaders that he would step down by the end of the year, according to a presidential spokesman, Ahmed al-Sufi. Saleh had earlier rejected such a proposal, making a more limited concession of promising not to run for re-election when his term ends in 2013.

But the opposition said the new offer was too little, too late.

"The president's statements are just another political manoeuvre," said chief opposition spokesman Mohammed al-Sabri. "What was acceptable yesterday is not acceptable for us today.

"There is only one option, that the president announces his resignation and hands over power. Only then can we meet with the president to agree on transferring power," he said.