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The pitched battle between President Ali Abdullah Saleh's forces and one of Yemen's oldest and largest tribal confederations has escalated to a level unseen in the capital for nearly half a century, with both sides bringing in reinforcements.

Gen. Ali Mohsen al-Ahmar, a top military leader who defected in March, deployed about 1,000 of his troops against President Saleh's forces for the first time Wednesday night to back up fighters from the Hashid tribal confederation, the Wall Street Journal reported. The confederation is led by Sadiq al-Ahmar (not related to the general). Sheikh Ahmar, along with his brothers, presents one of the biggest challenges to Mr. Saleh's grip on power.

Saleh, for his part, has deployed US-trained-and-funded counterterrorism forces against Ahmar's tribal fighters, according to the Journal, though the US says it has no evidence that Saleh has used the troops – intended to fight Al Qaeda in the Arabian Peninsula – against his political opponents.

Abdullah al-Faqah, a professor of politics at Sanaa University, described the battle as the worst the capital has seen in 50 years, Time magazine reports.

"What we're witnessing now is a battle between the two most powerful families in Yemen, a conflict that has been brewing for several years [... ]Saleh's stubbornness has come to its head," says Abdullah al-Faqah, professor of politics at Sana'a University. "This was a foolish fight for him [Saleh] to pick ... we're now witnessing the worst violence in Sana'a since the civil war in the late 1960's."

Last week, Saleh issued a warrant for Sadiq al-Ahmar's arrest, which the sheikh has so far ignored, calling the president a liar and vowing that he would leave the country 'barefoot'," Time reports. His brother Hamid al-Ahmar, the founder of one of the opposition parties, has set himself up as a potential successor to Saleh. Gen. Ali Mohsen al-Ahmar is not related to Sadiq al-Ahmar's family, but is rather a half-brother of Saleh.

The Journal reports that the general's decision to back the Ahmars "holds potential to move the country toward a sustained civil war, or urge it to a more decisive end" if Saleh scales back his violence to prevent being held responsible for massive casualties. Ahmar has an estimated 40,000 troops as well as heavy artillery at his disposal, while Saleh has some 50,000-60,000 troops.

As Saleh attempts to drive out the tribal fighters who have flocked to Sanaa, he is also waging battles in the southern city of Taiz, where peaceful protests were held for several months before being brutally dispersed this week, and Zinjibar, which Saleh's forces this weekend ceded to Islamist militants who they claim are linked with Al Qaeda in the Arabian Peninsula.

His grip on power is slipping as his forces are spread more thinly, and doubts are growing that a transfer of power can still be negotiated. Saleh has already allowed three power transfer deals to collapse. The more likely scenario now is that he will be ousted, Time reports.

"The sizable cohort of Yemen's armed forces seeking Saleh's ouster – which includes Major General Ali Mohsin al-Ahmar, the President's half-brother and commander of 1st armored division – appears to be swelling. A statement read out by former defense minister Abdullah Ali Eliwa and signed by nine senior army officials accused Saleh of "handing Zanjibar to terrorists" in order to "frighten people that if he goes, Yemen will become Somalia."

While fierce fighting continues in the capital, with constant gunfire and shelling, the humanitarian situation facing its civilian residents is increasingly dire. Most work has stopped, stores are closed, and shipments of necessary goods – food and gas, among others – has stalled. Time reported that many residents are attempting to flee and those who remain are hoarding all of their supplies and money.