Is this the end for Yemen president Saleh?

Yemeni president Ali Abdullah Saleh is looking ever more isolated after Barack Obama yesterday called for him to "move immediately on his commitment to transfer power".

Obama's words came as violence sharply escalated in the restive Middle Eastern country, fanned by Saleh's refusal last Sunday to sign an agreement which would have seen him step down from his 33-year rule in 30 days' time.

This is the third time Saleh has reneged on his promise to sign the deal, which was mediated by the Gulf Co-operation Council and backed by the US. Hillary Clinton reacted by saying that the US was "deeply disappointed", and accusing Saleh of "turning his back on his commitments and disregarding the legitimate aspirations of the Yemeni people".

Clashes broke out on Monday following an alleged attempt by government forces to break into opposition member Sadiq al-Ahmar's house in the capital Sanaa. Ahmar, who is leader of the Hashid tribe to which Saleh also belongs, responded violently, seizing several key buildings, including the interior ministry.

Although Ahmar pledged his support to the protesters back in March, until Monday he had refrained from involving himself in the mounting calls for Saleh to step down.

This sudden shift has been widely hailed as a sign that the stalemate that has gripped the country for weeks has broken - and not necessarily for the better.

Writing in the New York Times, Robert Worth said that there was a risk of a "broad factional conflict in which both sides are heavily armed", while an editorial in the Washington Post said that Saleh's final "absurd manoeuvre" had put Yemen on the "brink of civil war".

While the official call is still for an organised transfer of power, experts believe that the president has had his last chance. "We're not at this point relying on a change of heart on the part of Saleh," an anonymous US administration representative told the Post.

"It does appear to me that Saleh has finally overreached himself," Jane Kinninmont, senior research fellow at Chatham House's Middle East and North Africa Programme, told The First Post. "He has lost his Gulf allies, and by allowing forces to fire on a key tribal leader he has taken a very big risk.

"It's not yet clear what the endgame will be," she added, "but there is a high risk of further violence. It is basically going to come down to what happens internally. Clearly Saleh is not willing to step down at present but it may be that he will eventually be forced to by the evolving military situation."

Heavy fighting, including machine-gun fire and mortar shells, continued today in Sanaa as Saleh's troops and Ahmar's forces struggled to gain control of key government buildings. Saleh has ordered Ahmar's arrest, while Ahmar has reportedly scoffed: "Neither him nor [his] friends [are] good enough for that."

So far over 50 members of the opposition are said to have been killed in this week's fighting. A further 28 people died during an explosion at a weapons storage building in the capital. Opposition members are saying this is a lie to cover up deaths from the shelling of residential areas. According to Tom Finn in the Guardian, "lots of Yemenis in the old city [are] packing up their stuff and leaving for their villages. There seems to be a mass exodus."

The conflict may come to a head sooner than Saleh wants, however. Yemen's economy is apparently on the edge of collapse following months of unrest, and with the tribes finally mobilising against the beleaguered president, he may find himself unable to continue to pay his supporters. ·