Just weeks after the UN said Eritrean soldiers may be on the ground in Yemen, Sudan has reportedly sent hundreds of additional troops to fight for the Saudi-led coalition.

The apparent arrival of reinforcements from Saudi Arabia's unsavory allies — both Eritrea and Sudan have dismal human rights records — not only has the potential to plunge Yemen even deeper into chaos, but it puts the United States, a de facto member of the coalition, in the extremely awkward position of backing a side that includes fighters from at least one nation that it has repeatedly criticized for committing atrocities.

The Saudi-led alliance, which officially includes at least nine countries, has been fighting in Yemen since late March in response to the country's Shia Houthi rebels and their allies ousting the president and seizing power in most of the country. In mid-October, local sources confirmed to VICE News that several hundred Sudanese troops had arrived in the strategically important port city of Aden. On Monday, the French news agency AFP cited Yemeni officials as saying that 400 an additional Sudanese soldiers have landed in Aden, joining 500 that were already there.

Though VICE News could not immediately confirm the accuracy of the troop figures, it is clear that Sudanese forces are present on the ground in Yemen. The United Arab Emirates (UAE), which for years has filled its ranks with foreign nationals and enlists the assistance of private sector security contractors, has been mum about the makeup of its forces in Yemen.

Several human rights and diplomatic sources, speaking on condition of anonymity, have told VICE News that — like UAE's overall labor force — the country's forces include soldiers from outside the Gulf, including several Asian countries. Sudan's backing for the Saudi-led coalition, meanwhile, has been tied to Gulf financial assistance, including the extension of loans.

Late last month, a UN expert panel that monitors both Somalia and Eritrea reported that Eritrea has entered into an arrangement with both Saudi Arabia and the UAE that "involved allowing the Arab coalition to use Eritrean land, airspace and territorial waters in it's anti-Houthi military campaign in Yemen."

The monitoring group found that Eritrea's reported compensation, including money and fuel supplies, would violate a 2009 Security Council resolution that imposed an arms embargo on the country and financial sanctions and travel bands on its leaders.

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The monitoring group also reported that several well-placed sources, including a "former high ranking Eritrean official" had told them that "some 400 Eritrean soldiers were embedded with the United Arab Emirates," in Yemen. The UN experts said that this too represented a violation of the 2009 resolution.

The Eritrean government, considered one of the most oppressive in the world, practices forced conscription, which often sees citizens involuntarily added to military ranks, where they may remain for decades. These draconian policies, among others, are often cited for mass exodus of hundreds of thousands of Eritreans to Europe as part of the continent's ongoing refugee crisis.

The presence of Sudanese, and possibly Eritrean forces, puts the United States — Saudi Arabia's principal Western backer — in a bind. Washington has led criticism against the Sudanese government in Khartoum, accusing it of continued human rights abuses in several regions. The country's president Omar al-Bashir, remains under indictment by the International Criminal Court (ICC) for genocide and other crimes.

The US has, since the very start of the Arab-led intervention in Yemen, supplied the Saudis with vital logistical and intelligence support, including what it terms "targeting assistance." At the core of their logistical efforts are multiple refueling sorties that take place over Saudi Airspace.

According to US Central Command (CENTCOM), US tankers offloaded fuel to at least 2,139 aircraft between April 3 and October 23 — on average more than 10 planes per day. According to the UN, that majority of the more than 2,355 civilian killed since late March have died as a result of coalition airstrikes.

'While we don't know which units have been sent to Aden, what we do know is that they come from army with an appalling pedigree.'

US officials routinely disparage the Sudanese government. In September, Samantha Power, Washington's ambassador to the UN, criticized China for inviting Bashir, tweeting that Beijing "disregards International Criminal Court work on justice for victims of Bashir's abuses." Yet the US now finds itself providing explicit support for a coalition that counts Bashir among its collected leaders and top troop contributors.

On October 22, the UAE-owned newspaper The National cited a Yemeni official in a report that said the Sudanese troops deployed in Yemen are "specialists in combat on mountain terrain, and will be used to help liberate Ibb and Taez, both mountainous provinces."

Though details remain sparse on where and when the Sudanese forces will fight, reports saying they would be deployed in Yemen's mountain areas raised additional concern among human rights officials.

"The Sudanese army's approach to fighting in the mountains is clear," said Akshaya Kumar, Human Rights Watch's deputy United Nations director and an expert on Sudan. "In Darfur's Jebel Marra Mountains, South Kordofan's Nuba mountains and Blue Nile's Ingressana Hills, HRW researchers have found that its troops have ignored the laws of war and abused civilians with impunity."

"While we don't know which units have been sent to Aden, what we do know is that they come from army with an appalling pedigree," added Kumar.