Thick smoke rises from buildings across Yemen's capital Sanaa on Sunday...

...amid reports of advances by Shi'ite Muslim Houthi rebels.

Residents say the group has taken over several government sites, including the prime minister's buildings, an army command center, and the state TV station.

Yemen's Prime Minister Mohammed Salem Basindwa has submitted his resignation to the president.

Last month, the Houthi rebels began a wave of demonstrations to protest the Sunni-dominated government...

...that later escalated into armed clashes.

They had been due to sign a deal brokered by a U.N. special envoy, which was intended to end the fighting and pave the way for a new government within two weeks.

But it's unclear where they go from here.