Yemeni authorities on Wednesday accused Shiite rebels of "atrocities" in the northern city of Amran, which they seized in a major advance towards the capital.
In the thick of a bumpy political transition, Yemen is also grappling with an Al-Qaeda threat as well as a separatist movement in the south.
Amran, 50 kilometres (30 miles) north of Sanaa, has since February been the scene of fighting between troops and Huthi rebels, as well as tribes on both sides, as the rebels advanced from their mountain strongholds towards the capital.
Home to an estimated 120,000 people, the city fell into rebel hands on Tuesday after a three-day battle which has uprooted some 10,000 families, according to the Red Crescent.
By seizing Amran, the rebels have made a major advance towards the capital, posing a threat to President Abdrabuh Mansur Hadi's government.
Huthi Shiite rebels, known as Ansarullah, "stormed the headquarters of the 310th Armoured Brigade, looted weapons and equipment there, and killed a number of soldiers and officers," said Yemen's Supreme Security Committee, quoted by state news agency Saba.
It said they had also taken over government headquarters in the town.
A presidential committee which has negotiated several ceasefires with the rebels said Ansarullah had violated an agreement for the army to withdraw from the city.
Under the deal, the military police was to replace soldiers of the 310th Armoured Brigade as rebels withdrew from government headquarters they had seized.
But the rebels "did not comply with the agreement and attacked the brigade's headquarters and committed terrifying atrocities," it charged.
The security committee held the rebels responsible for the safety of the armoured brigade's commander, General Hamid al-Qushaibi, and "all soldiers and officers" they have captured.
Residents told AFP that rebels had also arrested supporters of the Sunni Islamist party Al-Islah, whose gunmen fought alongside the army.
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But the city was quiet on Wednesday, residents said, and authorities have so far not announced any offensive to drive out the rebels.
- Hadi in Saudi -
Amran's provincial governor, Mohammed Saleh Shamlan, denied media reports accusing him of having handed Amran city over to the rebels.
"This information is baseless," he told Saba.
As Ansarullah, secured their grip on Amran, Hadi made a surprise visit to Sunni-dominated Saudi Arabia Tuesday to discuss the escalation, according to local media.
He met with King Abdullah, whose country accuses Shiite-dominated Iran of supporting the Huthi rebels.
The rebels have been battling the government for years from their Saada heartland, complaining of marginalisation under former president Ali Abdullah Saleh, who stepped down in 2012 after a year-long uprising and was replaced by Hadi in a Saudi-sponsored deal.
Saudi Arabia itself has also launched operations against the rebels, attacking them by land and air since late 2009.
The rebels say a federalisation plan agreed in February after national talks as part of a political transition would divide Yemen into rich and poor regions.
They seized areas of Amran province in fighting with tribes that killed more than 150 people.
The Huthis are suspected of trying to expand their sphere of influence as Yemen is split into six regions, advancing from their mountain strongholds in the far north.