Airstrikes struck Yemen's capital, Sana'a, overnight and into Monday with witnesses saying the bombings hit Houthi rebel positions.
A Saudi-led coalition that has launched airstrikes against the Houthis in Yemen since March 2015 is the only force known to conduct strikes on Sana'a.
Robert Mardini, Middle East regional director for the International Committee of the Red Cross, said "the night was tough" in the city with massive clashes that included airstrikes and artillery. He reported Yemenis were trapped in their homes and that an ICRC medical warehouse was hit.
The violence comes as an alliance between the Iran-backed Houthis and former President Ali Abdullah Saleh appears to have fallen apart.
The two sides joined ranks three years ago and swept across Sana'a, forcing the country's internationally recognized President Abdu Rabu Mansour Hadi to flee the country and seek the military intervention led by Saudi Arabia.
The Houthis' political office accused Saleh of staging a "coup" against "an alliance he never believed in" after he offered to initiate talks with the Saudi coalition. In a televised speech, Saleh asked for an end to the siege on Yemeni ports and offered, in exchange, to "turn a new page" and "deal with them in a positive way."
Clashes between fighters loyal to Saleh and the Houthis first erupted last week when Saleh accused the rebels of storming his giant mosque in Sana'a and attacking his nephew, the powerful commander of the special forces, Tarek Saleh.
David Mack, a former U.S. diplomat, has dealt with Saleh who was removed in 2012 after more than 30 years in office. Now with the Middle East Institute, Mack described Saleh as "one of the smartest political operatives in the Arab world," and someone who felt he was unjustly pushed from power by members of the Gulf Cooperation Council backed by the United States.
"He still wanted to have one chance to restore his reputation, which has been terribly, terribly damaged during the past decade," Mack told VOA. "He’s gone from having the reputation of being this supremely capable political operator within Yemen to being somebody who was just out for personal power and was prepared to see tremendous suffering of Yemeni people in the process."
United Nations Secretary-General Antonio Guterres on Sunday urged warring parties in Yemen to stop all ground and air assaults. The U.N. said the sharp escalation of armed clashes and airstrikes in Sana'a has already resulted in dozens of deaths and hundreds of injuries, including civilians.
Fighting is restricting the movement of people and life-saving services within Sana'a city. Ambulances and medical teams cannot access the injured and people cannot go outside to buy food and other necessities, the U.N. said. Aid workers are unable to travel and implement critical life-saving programs at a time when millions of Yemenis rely on assistance to survive.
Yemen is one of the Arab world's poorest countries and has become a humanitarian catastrophe with more than 10,000 people killed in the fighting and at least 2 million displaced. A cholera outbreak has infected nearly 1 million people.