WASHINGTON: The United States is concerned that tribal rivalries are complicating efforts to reach a power transfer deal in Yemen and believes al Qaeda is trying to exploit instability there, senior US officials said on Saturday.

The United States remains in close contact with European and Gulf allies and continues to review options to increase pressure on Yemeni President Ali Abdullah Saleh to sign an agreement to step down, officials said.

The US government weighed in cautiously on Saturday even as Yemen's government and armed tribesmen demanding Saleh’s ouster agreed to end clashes that had brought the poor Arabian Peninsula country to the brink of civil war. The truce did not resolve the country's wider political crisis.

The latest violence, pitting Saleh loyalists against members of the powerful Hashed tribe led by Sadeq al-Ahmar, was the bloodiest since pro-democracy unrest erupted in January and was sparked by Saleh's refusal to sign a Gulf-brokered power transfer deal.

The fighting raised the prospect of chaos that could benefit the Yemen-based branch of al Qaeda and threaten neighboring Saudi Arabia, the world's biggest oil exporter.

“We are very concerned that the unsettled situation in Yemen is bringing longstanding tribal rivalries to the surface, which is further complicating the process of reaching an agreement on an orderly transfer of power,” one senior official said, offering the US position on condition of anonymity.

“Tribal as well as extremist elements are attempting to exploit the current instability in order to advance their own parochial interests.”

While US support for Saleh has eroded, Washington also has serious misgivings about the wealthy and powerful Ahmar clan and considers it unlikely to help bring about sweeping reform should it gain further clout, the official said.

The United States, which has had little sway in Yemen where tribal allegiances predominate, has signaled to Saleh that he must cede power by warning it may review its aid. But experts say cutting assistance would only reduce US influence further in a country crucial to American counter-terrorism efforts.

Still, President Barack Obama has faced criticism for not acting more assertively against Saleh, a longtime US ally.

Al Qaeda's Yemen-based wing, already topping US security concerns, is expected to step farther into the spotlight after US forces killed Osama bin Laden in Pakistan this month.