SANAA: Shiite militia clashed with Yemen's army and fired on a convoy carrying the prime minister Monday as pressure mounted on the embattled government of President Abdrabuh Mansur Hadi.

The militiamen, known as Houthis, opened fire on the convoy carrying Prime Minister Khalid Bahah in the capital Sanaa but he escaped unharmed, Information Minister Nadia Sakkaf said.

The alleged attack came after fierce clashes broke out near the presidential palace early Monday and following a meeting with Hadi at his residence in western Sanaa.

The U.S.-backed leader's chief of staff was abducted Saturday as the Houthis press for concessions in the writing of a new constitution.

The president's office issued a statement calling for an "immediate cease-fire" but fighting continued.

Yemen has been wracked by unrest since the Houthis seized control of Sanaa in September, raising fears the strategically important country bordering Saudi Arabia could collapse into a failed state.

Shelling and gunfire could be heard as smoke rose over parts of the city, and residents fled many neighborhoods.

Witnesses said the fighting erupted early Monday after the militia deployed reinforcements near the presidential palace.

The military presidential guard sent troops onto the streets surrounding the palace and outside Hadi's residence.

A security official said the army intervened when the Houthis began to set up a new checkpoint near the presidential palace.

But a prominent Houthi chief, Ali al-Imad, accused the presidential guard of provoking the clashes, in a statement on his Facebook page.

"Hadi's guard is trying to blow up the situation on the security front to create confusion on the political front," he said.

There were no immediate reports of heavy casualties but a security source said two Houthi militiamen were wounded in the clashes and two pupils hurt when a shell landed in a school.

Tensions have been running high in Sanaa since the Houthis abducted Hadi's chief of staff, Ahmed Awad bin Mubarak, in an apparent bid to extract changes to a draft constitution that he is overseeing.

Mubarak is in charge of a "national dialogue" set up after veteran strongman Ali Abdullah Saleh was forced from power in February 2012 following a year of bloody Arab Spring-inspired protests.

The Houthis said they had seized the top aide to prevent the violation of a U.N.-brokered agreement they reached with Hadi, which provided for the formation of a new government and the appointment of Houthi advisers to the president.