Yemen's government troops have been battling Houthi rebels for a third day in the capital Sanaa after the collapse of ceasefire talks, officials and residents say.
Explosions were heard on Saturday near the Interior Ministry in the northern part of Sanaa, a day after the Shia fighters began shelling the state television building as they advanced into the city.
Khaled Hammadi, a journalist based in Sanaa, told Al Jazeera that heavy smoke was hanging over the area of the state TV building on Saturday.
"Fighting is expanding hour by hour and the Houthi rebel fighters are advancing into neighbourhoods near important areas," he said.
Meanwhile, reports from Sanaa said the Yemeni army was commencing a counterattack on the Houthis with back-up from the Republican Guard unit.
Hakim al-Masmari, the editor-in-chief of the Yemen Post, told Al Jazeera that Saturday's clashes have been the fiercest since fighting began on Thursday. He said that dozens of Houthi fighters were killed in Sanaa in the latest round of fighting.
He added that there could be a truce between the warring sides later in the day.
At least a 123 fighters have been killed over the past three days, according to a tally by the Associated Press news agency.
Thousands of Houthis have been staging protests in Sanaa for more than a month now, besieging ministries and blocking the road to the main airport.
|Clashes in Sanaa kill dozens
Fighting in Sanaa had become so intense that by Friday international airlines suspended flights in and out of the nearby airport.
The UN has failed to mediate a peace deal between the warring sides.
Jamal Benomar, the UN envoy to Yemen, left the northern city of Saada on Friday after trying to mediate a deal that could pave the way for a new government and more political representation for the Houthis.
Clashes have raged on the outskirts of Sanaa for days, with dozens of deaths reported.
Earlier, Al Jazeera's Hashem Ahelbarra, reporting from Sanaa, said the country "is just a few hours" from plunging into a civil war as the capital is divided along sectarian lines, with one half run by Sunnis and the other by Houthis".
"The Houthis have control over most of the north of the country - from Saada to the gates of Sanaa," he said.
"They have thousands of fighters and some military commanders and members of the former regime with them. And if in the coming hours they decide to control Sanaa, they can definitely control the capital.
"If the Sunnis decide to join the fight against the Houthis, it's definitely going to be civil war in the country [...] It's either peace or war."
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 in Yemen has been rejected by both 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 for the formation a new government of technocrats, a reduction in fuel prices and more political representation. In exchange, the Houthis would pull out of Sanaa and put an end to their civil disobedience campaign.
President Abd-Rabbu Mansour Hadi faces a tough choice: he is from the south and northern Yemen is not his power base. 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 Houthis take control of 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.
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