Jamie McGoldrick of the UN OCHA aid agency said at least 25 air strikes had hit the city and “people are bracing themselves for more.”
Red Cross official Robert Mardini wrote on Twitter: “The night was tough in Sanaa.
“Massive urban clashes with heavy artillery & airstrikes. Yemenis stuck in their homes, too scared to go out. Reduced access to water, health care, food & fuel. Increased vulnerability of Yemenis put in harm’s way.”
The Red Cross said at least 234 people had been killed since Wednesday in six days of infighting between Saleh loyalists and Houthi militias. Clashes between the former allies erupted last week after the groups battled over control of Sanaa’s biggest mosque.
The two forces had been fighting against the current Saudi-backed Yemeni government, but the alliance broke down after Mr Saleh switched his allegiance to Saudi Arabia and abandoned the Houthis.
But Houthi fighters were quick to clamp down on a pro-Saleh uprising in the capital and shot Mr Saleh dead as he tried to flee to Saudi Arabia on Monday.
From Cairo, Arab League Secretary-General Ahmed Aboul-Gheit denounced Mr Saleh's “assassination” at the hands of “criminal militias,” warning that the humanitarian crisis in the country could worsen as a result.
But Iran, which supports the Houthis, said that a Saudi plot had been “nipped in the bud,” declaring that Riyadh, “on behalf of the US and the zionist [Israeli] regime, has a very important role in creating insecurity.”