Warplanes from a coalition led by Saudi Arabia have bombed the stronghold of Yemen's Houthi fighters with strikes targeting arms depots and a number of military camps controlled by the group.

Air raids early on Friday targeted Saada, the stronghold of the Houthis, and focused on locations along Yemen's porous border with Saudi Arabia.

Sources told Al Jazeera the air raids struck arms depots in the Malaheez and Hafr Sufyan regions of Saada and an army camp used by Houthi fighters in Kitaf.

Who are the Houthis?

- The Houthis belong to the Zaydi community, a moderate Shia sect that makes up about a third of Yemen's population.

- Their rebellion was launched in 2004 in the northern town of Saada.

- Their initial demand was an end to political and economic marginalisation in the north.

- Another objective was allegedly to restore the rule of Shia imams, who ruled Yemen for nearly 1,000 years.

- They now say they are fighting for more rights for all Yemenis, saying people in both the north and south are being wronged by the country's rulers.

- The Houthis participated in the 2011 uprising and gained political acceptance in Yemen. But soon they broke away from the rest of the political forces that deposed Ali Abdullah Saleh.

- They resumed military action and have in the last six months taken control of Sanaa and other parts of Yeme

The air campaign also targeted the al-Sana army camp in the Arhab region northeast of Sanaa, run by commanders loyal to former President Ali Abdullah Saleh, the Tariq Air Base in Taiz, central Yemen; and the presidential compound that Houthis seized in the south of the capital last month.

Residents said explosions echoed throughout the capital, creating panic and hysteria.

"Whenever a plane flies over our home and is met by anti-aircraft gunfire, my three children run to a corner and start screaming and crying," Mohammed al-Jabahi, a Sanaa resident told the Associated Press.

So far, at least 39 civilians have been killed in Operation Decisive Storm, officials at the rebel-controlled health ministry in the capital said.


Sanaa residents: 'The war is really here'


Twelve of the victims died when surrounding residential areas were hit in a raid against a military base north of the capital, the officials told the AFP news agency. Amnesty International said six children were killed on Thursday.

Saudi Arabia and a coalition of more than 10 countries view the Houthis takeover as an attempt by Iran to strenghten its influence in the region. 

The kingdom has deployed about 100 fighter jets, and 150,000 soldiers are standing by near the Yemeni border.

Iran and the Houthis deny that Tehran arms the rebel movement, though it says it provides diplomatic and humanitarian support.

Iranian Foreign Minister Mohammed Javad Zarif said in a statement on Friday that Tehran was "ready to cooperate with its brothers in the region, to make it easier for different groups in Yemen to have dialogue to protect the integrity and facilitate restoration of stability the security in that country."

According to defence officials in Washington, the US is providing refuelling tankers and surveillance flights for the Saudi operations, and there are several US troops working in the operations center to help with coordination.

The White House has said the US was not joining with direct military action.

The Houthi leader, Abdel-Malek al-Houthi, accused the US, Saudi Arabia and Israel of launching a "criminal, unjust, brutal and sinful" campaign aimed at invading and occupying Yemen.

"Yemenis won't accept such humiliation," he said in a televised speech on Thursday night, calling the Saudis "stupid" and "evil."


Timeline: The rise of Yemen's Houthi rebels


In a seperate development, at least 21 Houthis were killed near the coastal city of Aden on Friday when an anti-Houthi militia targeted their vehicle.

Witnesses told the AFP news agency that a "Popular committee" opened fire on the Houthis who were travelling from Lahj province towards Aden.

Popular committees operate across much of Yemen, often working alongside the military in defending state buildings from attack and manning checkpoints.

The Houthis descended from their heartland in the country’s north last year, fighting their way south towards Sanaa and defeating tribal and military rivals along the way.

Earlier this year, they put Hadi, the elected president, under house arrest, and appointed Mohammed Ali al-Houthi, a cousin of the group's leader, as the new president.

Hadi, who left the country on Thursday, had struggled to reassert his authority since escaping house arrest and fleeing to Aden last month.

Source: Al Jazeera and agencies