Meanwhile, Yemen's Houthi rebels said they had attacked and hit a military air base in southern Saudi Arabia with a drone, but the Saudi-led coalition fighting the group said it had intercepted and destroyed an armed drone fired into the kingdom.
At least 65 fighters have been killed during the past two days as Yemen's Houthi rebels make gains in their advance towards oil-rich Marib province.
The casualties over the past two days include some 26 loyalist forces, among them four officers, the Yemeni government sources said on Sunday. The Iran-backed Houthis rarely disclose their own losses.
The militants have taken full control of the northwest Kassara battlefront and made progress on western frontlines despite airstrikes by the Saudi-led coalition which backs Yemen's government.
Marib and its surrounding oil fields make up the last significant pocket of government-held territory in the north, the rest of which is under rebel control, including the capital Sanaa.
With the Houthis taking control of the Kassara front, the battles have moved to the Al Mil area, which is a mere six kilometres (four miles) from the centre of Marib and its population centres, they said.
However, mountains around Al Mil remain a formidable barrier to the rebels who launched a fierce campaign for Marib city in February.
The government sources said the Houthis had poured in hundreds of reinforcements in recent days to achieve the gains, resorting to motorbikes after the coalition targeted their military vehicles.
Saudi-led coalition says it has destroyed Houthi armed drone
Yemen's Houthi rebels said on Sunday they had attacked and hit a military air base in southern Saudi Arabia with a drone, but the Saudi-led coalition fighting the group said it had intercepted and destroyed an armed drone fired into the kingdom.
A Houthi military spokesman said on Twitter the group had targeted the King Khalid air base in Khamis Mushait with a drone.
There was no Saudi confirmation of any hit.
The military coalition led by Saudi Arabia, which intervened in Yemen against the Iran-aligned Houthis in 2015 after the Houthis captured the capital Sanaa, has said it intercepts the majority of Houthi missile and drone attacks.
The Houthi movement, which holds most of north Yemen, has kept up cross-border attacks on Saudi Arabia and a ground offensive in Yemen's Marib region at a time the United States and the United Nations are pushing for a ceasefire agreement.
Riyadh has proposed a truce, but the Houthis reject the offer, saying they want the full lifting of a sea and air blockade which allows the Saudi-led coalition to restrict import s into Yemen.
The conflict, seen in the region as a proxy war between Saudi Arabia and Iran, has killed tens of thousands of people and pushed millions to the brink of famine in what the United Nations calls the world's worst humanitarian crisis. (
The loss of Marib would be a disaster for the Yemeni government and for Saudi Arabia its fall could also lead to a humanitarian disaster, as vast numbers of civilians displaced from fighting elsewhere have sought refuge in the area.
Around 140 camps have sprung up in the surrounding desert to provide basic shelter for up to two million displaced, according to the Yemeni government.