Yemen's Shiite rebels say they will announce a "constitutional declaration" Friday afternoon in a move that could signal their formal takeover of the Yemeni government.

The rebels, known as Houthis, have controlled the capital, Sanaa, since September. Their escalating dominance, including a siege of the presidential palace, prompted President Abed Rabbo Mansour Hadi to resign in January.

Hadi and his cabinet are being held under house arrest by the rebels — who gave a Wednesday deadline for Yemen's political parties to negotiate an acceptable way forward or else they would begin acting unilaterally.

Under Yemeni law, only the president can issue constitutional declarations.