A series of explosions have been heard across the Yemeni capital as Houthi rebels and government forces battle for a fourth straight day, despite the announcement of a UN-brokered agreement due to be signed later on Sunday.
Al Jazeera's Khaled al-Hammadi, reporting from the ground, said most of the violence happened in northern Sanaa.
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"The air force deployed helicopters and jets all over the city," said Hammadi . "There was a huge explosion and smoke right now hangs over Azal hospital in north Sanaa."
The fighting came after UN special envoy Jamal Benomar, who had held talks with Houthi leader Abdulmalek al-Houthi in the rebels' home province of Saada, announced late on Saturday that an agreement had been reached to end weeks of fighting and protests that have crippled the capital.
"This agreement shall be a national document that will advance the path of peaceful change, and will lay the foundations of a national partership and for security and stability," Benomar said in a statement.
The latest bout of violence is the worst in Sanaa since President Ali Abdullah Saleh was forced to step down in 2012 after a popular uprising against his rule.
Peter Salisbury, a journalist and political analyst in Sanaa, told Al Jazeera that there is the sense now that "even if the deal is signed on paper, realistically the fighting is not going to end anytime soon".
Residents reported sounds of heavy shelling throughout Saturday night near the headquarters of the first armoured division camp and close to the religious university of Iman.
Yemen's state TV headquarters in Sanaa had earlier been captured by the Shia rebels after coming under heavy shelling, while the country's Supreme Security Commission, chaired by President Abdrabuh Mansour Hadi, ordered a curfew in four areas north and west of the capital between 9pm and 6am.
More than 100 people have died in fighting since Thursday, sparked by weeks of protests and clashes. It also prompted the suspension of international flights to Sanaa and the interruption of broadcasts by state television.
Thousands of Houthis have staged protests in Sanaa for more than a month now, besieging ministries and blocking the road to the main airport.
The Houthis are a Zaidi Shia group whose traditional power base is in the north. They are demanding a new government and also more political power for their community.
The government's plans for a six-region federation have been rejected by the Houthis and the southern separatists.
ANALYSIS FROM OUR CORRESPONDENT
| Al Jazeera's Hashem Ahelbarra
The UN was trying to mediate a deal which included the Houthis demands of the formation a new government of technocrats, a reduction in fuel prices and giving Houthis more political representation. In exchange, the Houthis would have had to pull out of Sanaa and put an end to their civil disobedience campaign.
President Abdrabuh Mansour Hadi faces a tough choice: he is from the south and northern Yemen is not his power base. Most of the army’s top military commanders are Shias and he is worried they might defy his orders if he calls for war.
On the other hand, there is a growing frustration among the Sunnis. They say Hadi is weak, and that under his tenure more territory was lost to the Houthis. If the Shia rebels control Sanaa, Sunni tribes might call for his resignation.
And if fighting breaks out in Sanaa, a city divided along sectarian lines and armed to the teeth, it might be the worst on the Arabian Peninsula in modern history.
Explaining Yemen's political-military groups:
Houthis - Shia group also known as Ansarullah, or "Partisans of God", who have been at war with the government since 2004. They demand resignation of government, more political inclusion and access to the sea. Strongholds include Saada, al-Jawf and the Jeraf district inside Sanaa.
Government - A coalition that includes the former ruling General Congress Party and the opposition led by the Sunni Islah Party. The coalition was formed under an agreement brokered by the GCC which stipulated that in a country like Yemen consensus is the only way to avoid civil war.
Al-Islah (Reform) - Sunni Islamist party that draws support and membership from heavily armed Sunni tribesmen, and is instrumental in rallying support behind the army and the government. Present in almost all of Yemen. The Houthis have identified the party as its arch-enemy.
Al-Qaeda in the Arabian Peninsula - A merger of the Yemeni and Saudi branches of al-Qaeda. Seized large swathes of territory in the south and the southeast after the uprising in 2011. Launched many attacks on armed forces and central authority establishments. Its power bases are Shabwah, Abyan and Hadramawt.
The Southern Separatist Movement - Umbrella group that wants the south to break away from the north and reinstate the former Socialist state that existed until 1990. Led by Ali al-Beidh.
Source: Al Jazeera and agencies