Aden (AFP) - Fighting raged between Yemeni government forces and Iran-backed rebels Saturday as the UN voiced alarm at ceasefire violations but insisted that peace talks in Switzerland are still alive.

Loyalists were within 40 kilometres (25 miles) of the rebel-held capital as they pressed their advance against the Shiite insurgents, military sources said, in disregard of a UN-brokered ceasefire.

Pro-Hadi forces supported by a Saudi-led coalition were keeping up pressure in Nihm district, Sanaa province, after significant gains in Marib province, east of the capital.

The forces loyal to President Abedrabbo Mansour Hadi, and allied tribes in the area, have laid siege to Nihm's Fardha military base, northeast of the capital.

Despite the proximity, the roughly 40 kilometres separating Nihm from Sanaa is mostly rugged mountainous terrain.

On Friday, loyalists seized Hazm, capital of Jawf province, to the northeast of Sanaa.

By Saturday, they had extended their gains in Jawf, capturing Al-Ghayl and Al-Maton districts, according to sources in the pro-Hadi Popular Resistance militia.

The two areas fell after clashes between advancing forces and rebels and renegade troops loyal to former president Ali Abdullah Saleh, said Ameen al-Ukaymi, a tribal chief of the Popular Resistance.

Government forces are now heading west, toward the contiguous rebel strongholds of Amran and Saada provinces, which lie immediately to the north of the capital, Ukaymi said.

- UN 'deeply concerned' -

Military sources said pro-Hadi forces, which are supported by a Saudi-led Arab coalition, brought reinforcements to Hazm Saturday, including tanks and armoured vehicles.

The operations took place despite a ceasefire that has been repeatedly violated since it entered into force Tuesday, as UN-sponsored talks began in Switzerland.

The UN Yemen envoy was "deeply concerned at the numerous reports of violations of the cessation of hostilities," his office said late Friday.

Story continues

His comments came after the pro-government forces seized two northern towns and the rebels fired two ballistic missiles at Saudi Arabia.

He "urges all parties to respect this agreement and allow unhindered access for the delivery of humanitarian assistance to the most affected districts of Yemen," the statement said.

It was issued after a fourth day of rocky peace talks, during which the special envoy "held several sessions with the participants," the statement added.

The discussions "focused mostly on security issues in Yemen, in light of the alarming developments on the ground," it said, stressing that both sides had "renewed their commitment for a ceasefire."

"A coordination and de-escalation committee was created to strengthen adherence to the cessation of hostilities," the statement said.

The UN announced a first breakthrough in the talks Thursday, saying the sides had agreed to "allow for a full and immediate resumption of humanitarian assistance" in the flashpoint Yemeni city of Taez.

But a local relief group, the Humanitarian Relief Coalition, said no UN aid had reached the city, accusing rebels of blocking aid delivery to areas where Hadi loyalists are holed up.

On Thursday, pro-government forces and rebels completed an exchange of hundreds of prisoners in the southern province of Lahj.

Yemen's conflict began in September 2014, when the Huthis advanced from their northern strongholds to occupy the capital Sanaa.

It has escalated dramatically since Saudi-led air strikes against the rebels began in March, with more than 5,800 killed and more than 27,000 wounded since then, according to the UN.