The militants seized control of state media in fierce fighting that marked the biggest challenge yet to President Abed Rabbo Mansour Hadi by the rebels, known as Houthis, who swept down from their northern strongholds last year and captured the capital in September.

The violence threatened to undermine efforts by the U.S. and its allies to battle al-Qaida’s Yemeni affiliate, which claimed responsibility for the attack on a Paris satirical magazine this month and which Washington has long viewed as the global network’s most dangerous branch.

The Houthis and forces loyal to Hadi have been in a tense standoff for months and the two sides traded blame for the outbreak of violence Monday. Witnesses said heavy machine gun fire could be heard as artillery shells struck around the presidential palace. Civilians in the area fled as columns of black smoke rose over the palace and sirens wailed throughout the city.

Hadi, whose government has ceded control over nearly the entire capital, doesn’t live at the palace, and extra soldiers and tanks deployed around his private residence, which is nearby.

As fighting escalated, the convoys of Yemen’s prime minister and a top official affiliated with the Houthis came under fire, and rebel fighters took over Yemen state television and its official SABA news agency, Information Minister Nadia Sakkaf said.

“This is a step toward a coup and it is targeting the state’s legitimacy,” Sakkaf told The Associated Press.

Cease-fire negotiations continued throughout the day. By the end of the day, a tenuous truce appeared to be holding.

Yemen’s Western and international allies, including the U.S and Saudi Arabia, called for steps to implement and consolidate the cease-fire, expressing their support for Hadi.

The Houthis blamed Hadi for the escalation in hostilities, saying he reneged on a U.N.-sponsored agreement with the group in September that promised better representation on a committee to oversee the drafting of a new constitution.

The Houthis are seen by their critics as a proxy of Shiite Iran and are believed to be allied with former President Ali Abdullah Saleh, who ruled the country for more than three decades before he was ousted in 2012 after Arab Spring protests. While the militants deny any Iran link, their slogan, “Death to Israel, Death to America!” is a variation of a popular Iranian slogan often chanted by militants in Iraq.