Additionally, on Monday, a court began proceedings in absentia against Yahya al-Houthi, the brother of the al-Houthis' leader, Malik al-Houthi, on charges of spying for an undisclosed nation.

The charges, which al-Houthi denies, carry a maximum sentence of the death penalty.

'Unlawful'

Yahya al-Houthi, who lives in exile in Germany, told Al Jazeera that the court cases were unlawful, and the Houthis could put captured Yemeni soldiers on trial in retaliation.

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"First of all, it is not an ordinary court of law. It was established to apprehend and prosecute those who oppose the dictator," he said.

"It is in conflict with the Yemini constitution. Therefore, it is unlawful and its rulings are void.

"However, the relatives of the sentenced defendants may pressure the Houthi leaders to try the prisoners from the army who reek destruction, looting and committed crimes.

"The Houthis till now have captured more than 400 to 500 from the army troops, including high-ranking officers and soldiers."

Al-Houthi rebels initially began fighting for a return to autonomous rule in the north in 2004, but the conflict intensified last August when the rebels stepped up pressure on government forces.

The military responded with Operation Scorched Earth and Sanaa has vowed to crush the Houthis.

The Houthis, who are from the Zaidi sect of Shia Islam, say they have been politically, economically and religiously marginalised by the government, which they see as corrupt.

Thousands of people have died and tens of thousands have been displaced due to the conflict, according to the Red Cross and the UN.