ABIDJAN, Yemen: U.S. Secretary of State Hillary Clinton said on Tuesday the unrest in Yemen was a "major concern", as Al-Qaeda militants took advantage of a central government weakened by months of protests.

"The instability in Yemen is a great concern first for the Yemeni people, but also the region and the world," Clinton said during her visit to Ivory Coast.

"It remains a great challenge," she said at a press conference. "There has been agreement with respect to the way forward that has not been fulfilled."

A Gulf-mediated plan for Yemen calls for the election of a consensus president for a two-year interim period after which parliamentary and another round of presidential elections are to be held.

But Yemeni officials have said the unrest in one of the world's poorest countries could delay the presidential election planned for February 21.

"We regret that the president has yet failed to comply with his own commitment to leave the country and to permit elections," added Clinton, referring to embattled Yemeni President Ali Abdullah Saleh.

After a year of a brutal crackdown on mass anti-government protests which left hundreds of people dead dead, Saleh handed over power to his vice president until elections could be held while he remains honourary president.

The weakened central government has lost its ability to enforce control in outlying provinces where Al-Qaeda in the Arabian Peninsula has seized control of towns and cities in the restive south.

On Monday, officials said AQAP and its local affiliates had taken over a town close to the capital Sanaa, a major advance for the militants.

In the north, Shiites and Sunnis have been engaged in fierce battles in recent months, raising sectarian tensions and leaving dozens of people dead.

Vice President Abdrabuh Mansur Hadi is expected to be the sole candidate in next month's election.