Saudi forces have reportedly crossed the border into northern Yemen near a site where Houthi rebels have launched shelling attacks toward the kingdom.

The incursion marks the first time that Saudi Arabia's troops have entered Yemen since a Saudi-led coalition began an air campaign in March against the Houthis, who captured large swaths of Yemen earlier this year.

Footage showing the troops on the Yemeni side of the border with the Saudi province of Jizan emerged on Wednesday a day after authorities in Riyadh said a battle for the Houthi-controlled Yemeni capital of Sanaa was imminent. The news organization Al Jazeera, which is owned by the government of coalition-member Qatar, reported that military officials in Riyadh called the incursion of Saudi ground forces "temporary."

Forces aligned against the Houthis and their allied loyalists of Ali Abdullah Saleh, the country's former president, have made significant gains in recent weeks, expelling the rebels from the Southern port of Aden and pushing north toward strategic cities like Taiz, Yemen's third largest.

While Emirati special forces have reportedly taken part in some fighting in Yemen's south, Saudi soldiers have not, and the kingdom's commanders appeared loath to send their troops into conflict areas. Houthi rebels have repeatedly hit and in some cases killed Saudi forces guarding the border, however, and in one instance documented by a pro-Houthi television station, appeared to cross into Saudi territory.

Amid gains claimed by the coalition in Yemen, human rights officials have heavily criticized the civilian toll of the airstrikes it has carried out. According to UN figures, 73 percent of children killed in Yemen between April 1 and June 30 perished as a result of coalition airstrikes. More than 400 children have been killed since the initiation of the air campaign on March 26.

Human Rights Watch, Amnesty International, and more than twenty other groups have called for the United Nations' Human Rights Council to establish a commission of inquiry to investigate abuses committed in Yemen that would be similar to the body created after last year's war in Gaza.

Both sides of the conflict have been implicated in gross human rights violations. Indiscriminate shelling by the Houthis has reportedly claimed hundreds of lives, while coalition airstrikes have been blamed for killing dozens of civilians at a time in the coastal city of Mokha as well as Taiz. The UN's humanitarian coordinator Stephen O'Brien said last week that the coalition's aerial and naval bombardment of the Red Sea port of Hodeidah was a clear violation of international humanitarian law.

Adding to the evidence compiled against the coalition, Human Rights Watch on Thursday released a report outlining what it claimed was cluster munitions use earlier this year. Investigators from the group determined that coalition forces had dropped American-made cluster bombs in the northwestern province of Haija between late April and the middle of July, "killing and wounding dozens of civilians."

More than 100 countries have signed the Convention on Cluster Munitions, which bans the use and stockpiling of the weapons, though Saudi Arabia and the United States have not. Cluster bombs release hundreds of bomblets in order to cause blanket damage over a wide area. Some bomblets will often fail to detonate and remain on the ground as a latent explosive threat.

At four sites visited by Human Rights Watch, staffers found "unexploded sub-munitions or remnants of cluster munition rockets."

"Several of the attacks took place in or near areas with concentrations of civilians, indicating that the rocket attacks themselves may have been unlawfully indiscriminate in violation of the laws of war," said the organization.

Last year, VICE News visited parts of northern Yemen and found evidence of cluster munitions that were allegedly used by the Saudi government against the Houthis in 2009, at a time when the group was engaged in war against the government in Sanaa. Saleh, who governed Yemen until 2012, switched sides after losing power during the Arab Spring, throwing the weight of large parts of the country's armed forces alongside the Houthis, who hail from a Shia minority community in the country's north.

"The loss of civilian life in Hajja shows why most countries have made a commitment never to use cluster munitions," said Ole Solvang, senior emergencies researcher at Human Rights Watch. "These weapons not only kill or injure people at the time of attack, but the unexploded submunitions go on killing long afterward."

The American-supplied munitions are only part of Washington's support for the coalition. The US military has established a joint command center with Saudi Arabia that offers "logistical support, intelligence support and intelligence sharing, targeting assistance, and advisory support," according to a spokesperson for US Central Command.

While UN efforts to broker a ceasefire languish and the Saudi-led coalition shows no sign of relenting, the targeting support offered by American personnel has come under increased scrutiny. American defense officials refuse to detail their role in specific incidents, including a July 24 strike in Mokha that killed at least 65 civilians.

Al Qaeda's branch in Yemen, long the top concern for American officials in the country, has meanwhile capitalized on the chaos. Al Qaeda militants currently control Mukalla, capital of Hadramawt, Yemen's largest province. While the Saudi-led coalition has yet to launch any attacks against al Qaeda — the Sunni extremist group is also a foe of the Houthis and might be seen by the kingdom's forces as assisting in their effort — American drone strikes have sporadically hit suspected militant sites.

On Thursday, Yemeni government officials cited by AFP said an alleged US drone strike in Mukallah left five members of al Qaeda dead. VICE News could not confirm the attack or the identities of those killed.