Yemen's government in exile said Friday that it would join UN-mediated peace talks next week, as Iran-backed rebels were accused of killing 20 civilians at a busy market.

The Saudi-led coalition battling the Iran-backed Shia Houthi insurgents meanwhile sent new reinforcements over the border into Yemen in preparation for an offensive to retake the capital Sanaa.

Coalition warplanes struck an arms depot in Sanaa, triggering powerful explosions that killed at least seven civilians and wounded 10, witnesses and medics said.

International rights groups have repeatedly voiced alarm at the heavy civilian toll in the conflict, which has killed more than 4,500 people since March, according to UN estimates.

The coalition sent more military vehicles and troops across the border into Yemen Friday to try to recapture more territory from the Houthis, who overran the capital a year ago and swept southwards across the impoverished country.

An AFP reporter at the Wadia border post between Saudi Arabia and Yemen saw at least 40 coalition military vehicles cross over into Yemen's oil-rich Marib province on Friday.

The vehicles were carrying Yemeni troops trained in Saudi Arabia as well as coalition troops whose nationality military officials in the area declined to specify.

Their arrival came as coalition warplanes killed seven rebels in an air raid Friday in eastern Yemen, where Apache helicopters have also joined the fight against the insurgents.

The strike targeted a military vehicle used by the rebels in the eastern province of Marib, while other air raids struck rebel positions in neighbouring Shabwa province, military sources said.

Hours later, the rebels fired Katyusha rockets at the government headquarters in Marib's provincial capital, witnesses said.

But several rockets landed on an adjacent market in Marib's city centre, killing at least 20 civilians and wounding dozens, according to medics and witnesses.

Friday's violence came hours after UN special envoy for Yemen, Ismail Ould Cheikh Ahmed, said that the exiled government and the Houthi rebels had agreed to take part in peace talks that "will convene in the region next week".

The country's government-in-exile confirmed in Riyadh on Friday that it had agreed to take part in UN-mediated peace talks next week in Oman.

Oman, the only Gulf Cooperation Council member not taking part in the coalition, had hosted talks between the rebels and a US delegation before a previous, failed round of UN-brokered negotiations took place in Geneva in June.

It is also the only GCC member which has always maintained good relations with Iran.

However, in the absence of an announcement from the rebels, Yemeni government spokesman Rajih Badi was unsure the rebels would attend.

The talks "may not take place", he told AFP.

The government of President Abedrabbo Mansour Hadi insisted that a rebel pullback from areas seized since last year -- as outlined by UN Resolution 2216 -- remained a precondition for negotiations.

The Saudi-led coalition backing Hadi has waged daily air strikes against the rebels, who are backed by troops loyal to former president Ali Abdullah Saleh.

Gulf Arab members of the coalition, in which the UAE is also playing a major role, have reportedly already sent thousands of heavily armed reinforcements to Yemen, mainly to Marib, since last week in preparation for the push on the capital Sanaa.

The exiled government has also announced that 10,000 Yemeni fighters are now ready to serve in a "national army being prepared to liberate Sanaa and other provinces".

Military sources in Marib said that the coalition's Apache helicopters were also taking part in the battle against rebels in the province.

"Preparations are ongoing for large-scale military operations to liberate the provinces of Marib and Jawf (in the north) in order to enter Sanaa," which the rebels seized a year ago, one of the Yemeni military officials told AFP.

The coalition has intensified raids against rebel forces since a deadly missile attack last week killed 60 Gulf troops, most of them Emiratis, in Marib province.

Since mid-July, pro-Hadi fighters, backed by the coalition, have recaptured main port city Aden and four other southern provinces.