Update at 11:55 a.m. ET
The Yemeni president's house is being shelled by Shiite rebels, and the presidential palace reportedly has been seized.
Nadia al-Saqqaf, the information minister, said on Twitter that President Abed Rabbo Mansour Hadi's house was being shelled by Shiite rebels whom Saqqaf called "militias seeking to overthrow the ruling system."
She said Hadi's home has been under attack since 3 p.m.
Reuters quoted an unnamed official as saying "the president is in the residence and he is fine."
The Associated Press is calling the "shelling ... a dramatic escalation in the violence that has gripped Sanaa since Monday."
Earlier, Col. Saleh al-Jamalani, the commander of the Presidential Protection Force, told the AP the Shiite Houthi rebels have seized the presidential palace in Sanaa, the capital. He called it a "coup." He said the rebels were aided by insiders and were looting arms depots on the grounds.
But the rebels said they had merely foiled attempts by another, unspecified group to loot the arms depot.
Hadi, the AP reported, wasn't at the presidential palace but was at his residence.
In a statement, a spokesman said the U.N. Secretary-General was "gravely concerned" about the situation. The U.N. Security Council was holding an emergency meeting over the events.
The Shiite Houthi rebels say they are fighting for the rights of Yemen's Zaydi Shiite Muslim sect. As we previously reported, the rebels seized large parts of the country, including the capital, last year. On Saturday, the rebels kidnapped Ahmed Awad bin Mubarak, Hadi's chief of staff.
The AP has more on the fighting:
"The latest spasm of violence followed apparently unsuccessful negotiations earlier in the day between Hadi and a representative of the Houthis at his residence.
"Also earlier Tuesday, Houthi fighters roamed the streets on foot and in pickup trucks mounted with anti-aircraft guns, manned checkpoints across Sanaa and near the prime minister's residence, and beefed up their presence around other key buildings, including the intelligence headquarters.
"The show of force came after they seized control of state media in Sanaa and clashed with Yemeni soldiers near the presidential palace on Monday. Heavy machine gun fire and artillery shells struck around the presidential palace and sent civilians fleeing as columns of black smoke rose and sirens wailed throughout the city."
Al-Jazeera explains the roots of the current crisis in Yemen and what it means for the country and its neighbors.