Last month also marked the anniversary of Saudi Arabia's entrance into the civil war on its southern border -- an intervention that has seen Riyadh and its allies get bogged down in a difficult battle with the country's Houthi rebels, who still control the capital Sanaa and much of northern Yemen.

Separately, a Reuters special report raised the possibility that al-Qaeda in the Arabian Peninsula, the terror group's influential Yemeni wing -- long the target of U.S. counterterror operations -- has gained ground in the shadow of the Saudi-led war, exploiting Yemen's security vacuum to consolidate its position in a string of coastal cities while building up an impressive treasury of plundered wealth.

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Much to the chagrin of some Arab allies, the United States has played a somewhat quiet role in the conflict over the past year. It has still provided the Saudi-led effort with intelligence, airborne fuel tankers, as well as advanced munitions.

Those armaments, according to the new HRW study, could be traced to a strike on March 15 in the crowded market in the village of Mastaba. Some 10 Houthi fighters were believed slain in the twin explosions — caused by two bombs dropped five minutes apart -- but dozens more civilians perished.

The incident drew condemnation from U.N. Secretary General Ban Ki-moon, who called for an independent investigation.

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Here's what HRW's researchers found:

Human Rights Watch conducted on-site investigations on March 28, and found remnants at the market of a GBU-31 satellite-guided bomb, which consists of a US-supplied MK-84 2,000-pound bomb mated with a JDAM satellite guidance kit, also US-supplied. A team of journalists from ITV, a British news channel, visited the site on March 26, and found remnants of an MK-84 bomb paired with a Paveway laser guidance kit. Human Rights Watch reviewed the journalists’ photographs and footage of these fragments.

When contacted by the New York Times regarding the rights group's report, a spokesman for the United States Central Command, or Centcom, put distance between the attack and American culpability.

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"We have consistently reinforced to coalition members the imperative of target analysis and precise application of weapons in order to identify and avoid structures and areas that, if struck, could result in civilian casualties," the spokesman said.

But the ongoing toll of airstrikes in Yemen -- some 3,200 civilians have died over the course of the war, the majority as a result of aerial attacks -- have led some governments elsewhere to consider bans on weapon sales to Saudi Arabia.

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"One of the deadliest strikes against civilians in Yemen’s year-long war involved U.S.-supplied weapons, illustrating tragically why countries should stop selling arms to Saudi Arabia," Priyanka Motaparthy, emergencies researcher at Human Rights Watch, said in a statement. "The U.S. and other coalition allies should send a clear message to Saudi Arabia that they want no part in unlawful killings of civilians."

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Last month, the Dutch parliament voted to halt weapons exports to the Saudis, citing continued humanitarian violations.

Meanwhile, as the Saudi-led campaign helped push the Houthis -- a faction loosely allied to Iran -- out of areas in the country's south, al-Qaeda's Yemeni wing, known by the acronym AQAP, and its tribal allies filled the security vacuum, as the map above shows. The militant group is linked to the deadly 2015 assault on the Paris offices of Charlie Hebdo, a satirical French newspaper, as well as numerous attempted bombings of international passenger flights.

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Reuters detailed the extent of their gains, which include key ports linked to Yemen's struggling oil and gas industry. The advance was made possible by the U.S. and Saudi distraction with the war against the Houthis, reports Reuters:

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A senior Yemeni government official said the war against the Houthis “provided a suitable environment for the … expansion of al Qaeda.” The withdrawal of government army units from their bases in the south, allowed al Qaeda to acquire “very large quantities of sophisticated and advanced weapons, including shoulder-fired missiles and armed vehicles.” As well, the coalition’s preoccupation with fighting the Houthis “made it easier for al Qaeda elements to expand in more than one area,” he said. “And this is why al Qaeda has today become stronger and more dangerous and we are working with the coalition now to go after elements of the group … and will continue until they are destroyed.”

In some areas, AQAP's fighters and proxies are amassing money looted from banks and extorted from various state companies. The article describes the port of Mukalla on the Gulf of Aden as the AQAP equivalent of the Syrian city of Raqqa, which is the de facto capital of the Islamic State jihadist organization. Some residents say life is more stable and peaceful under al-Qaeda's watch.

AQAP fighters, Reuters reports, have embarked on "a brazen campaign to shake down state-owned firms, including the national oil and mobile phone companies. AQAP uses the money it extorts to win favor among its subjects. Elisabeth Kendall, a Yemen scholar at Oxford University, calls it a 'Robin Hood' strategy."

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This has undeniably scary implications. "We may be facing a more complicated al Qaeda," a senior regional diplomat told Reuters, "not just a terrorist organisation but a movement controlling territory with happy people inside it."

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In September 2014, President Obama cited American counterterrorism strategy in Yemen as a model for the then fledgling U.S.-led mission against the Islamic State militants in Iraq and Syria. With Yemen torn asunder and al-Qaeda once more in the ascendance, it seems an unfortunate precedent to have invoked.