GWEN IFILL: Finally tonight, a sometime U.S. ally falls deeper into chaos.
Margaret Warner has the Yemen story.
MARGARET WARNER: Heavy fighting resumed today in Yemen's capital, Sana'a, after a cease-fire broke down between President Ali Abdullah Saleh government troops and tribal militia. It was the latest bloody twist in a four-month-long standoff between Saleh and three different groups trying to bring him down.
The militia fighters in Sana'a are loyal to the large, powerful Hashid tribe, led by a Saleh rival Sheikh Sadeq al-Ahmar.
SHEIKH SADEQ AL-AHMAR, Hashid Tribe (through translator): We will not allow Saleh to lead Yemen to civil war. He attacked our houses, and we are steadfast.
MARGARET WARNER: Elsewhere yesterday, government gunmen in the city of Taiz rained fire on another group of Saleh opponents, younger anti-government demonstrators.
And in southern city of Zinjibar, Yemen's air force pounded Islamist militants who took control there last Friday. Amid all this, the crumbling regime of President Saleh lurches onward, despite his repeated pledges to leave office.
Just five weeks ago, Saleh said publicly he'd agreed to abdicate under a deal brokered by the Saudi-led Gulf Cooperation Council and backed by Washington, but the next day he expressed defiant reluctance.
ALI ABDULLAH SALEH, president of Yemen (through translator): This is a coup. You call on me from the U.S. and from Europe to hand over power? Whom should I hand it over to, those who are trying to make a coup?
MARGARET WARNER: Saleh has ruled impoverished Yemen for 33 years. He faced threats even before Arab spring-style street protests erupted in late January.
They included rebellions in the north and south and an al-Qaida terror franchise headquartered in the sparsely populated eastern part of the country. This potent al-Qaida offshoot triggered ever-deeper U.S. involvement in Yemen and support for Saleh.
But this spring, after government forces began killing protesters, the U.S. began working with the Saudis to ease him out. Last week, after missing two agreed-upon deadlines to sign the agreement, Saleh balked a third time.
ALI ABDULLAH SALEH (through translator): We do not want any foreign intervention or solution from outside of Yemen. It is a Yemeni issue. We should go into a dialogue with all political sides. A solution will come from us, and not from the outside.
MARGARET WARNER: A day later, speaking in Paris, Secretary of State Hillary Clinton made her toughest statement yet on Saleh's status.
SECRETARY OF STATE HILLARY RODHAM CLINTON: We continue to support the departure of President Saleh, who has consistently agreed that he would be stepping down from power, and then consistently reneged on those agreements, turning his back on the commitments that he made, and disregarding the legitimate aspirations of the Yemeni people.
MARGARET WARNER: Now, in a country awash in weapons, those aspirations are on hold. Instead, fears abound that this nation of 22 million could disintegrate into chaos.