The head of Yemen's Shiite Huthi militia threatened Saturday to take control of the oil-rich Marib province, targeted by the group since it seized the capital and central areas three months ago.
"If official authorities do not assume their responsibilities, (we) will act to support the honourable people of Marib," Abdelmalek al-Huthi said in a televised address to supporters in Sanaa.
Yemen has been dogged by instability since an uprising forced longtime strongman Ali Abdullah Saleh from power in 2012, with Huthi rebels and Al-Qaeda seeking to fill the power vacuum.
The Huthis, also known as Ansarullah, overran Sanaa unopposed in September and have since advanced into mainly Sunni parts of the country.
They have been met with fierce resistance by Al-Qaeda and Sunni tribal fighters.
Huthi accused "certain" tribesmen of wanting to hand over Marib to "Al-Qaeda and the takfiris," a reference to the Islamist Al-Islah movement that has fought alongside Al-Qaeda's Yemen franchise against the Shiite group.
On Thursday, Sunni tribesmen ambushed a military convoy travelling between Marib and Sanaa and seized heavy weapons they claimed were destined for the Huthis, tribal sources and witnesses said.
Three soldiers died during ensuing clashes, according to Yemen's High Security Commission, which demanded late Friday that the tribesmen return the seized equipment.
On Thursday, almost 50 people were killed in a suicide bomb attack on a religious celebration by Huthi supporters in the mainly Sunni city of Ibb.
Twin attacks blamed on Al-Qaeda Saturday killed a soldier and a police sergeant in the eastern province of Hadramawt, security sources said.
The sergeant was killed by gunmen on a motorcycle as he returned to his home in Seyun, the province's second town, police said.
In neighbouring Shabwa province, one soldier was killed and another wounded when suspected Al-Qaeda militants ambushed their vehicle, a military source said.
Al-Qaeda in the Arabian Peninsula, considered by the United States to be the global jihadist network's most dangerous branch, has recently carried out a spate of attacks against Yemen's security forces.