Fresh evidence shows that military hardware that was supplied to US allies has been distributed in contravention of arms deals to militia groups, including UAE-backed separatists.

They are now using it to fight the Saudi-supported forces of the internationally recognized government, who are also armed with US weapons.

These new findings follow an exclusive investigation by CNN in February which traced US-made equipment that was sold to Saudi Arabia and the United Arab Emirates.

The weapons were being passed to non-state fighters on the ground in Yemen, including al Qaeda-linked fighters, hardline Salafi militias and the Iranian-backed Houthi rebels, the report found, in violation of arms sales law.

Following CNN's initial reporting, the Pentagon said it had launched its own investigation into the unauthorized transfer of US weapons in Yemen.

But more than half a year on and the situation on the ground only seems to have got worse.

Saudi Arabia has led a coalition, in close partnership with the UAE and including various militia groups, to fight Iranian-backed Houthi rebels in Yemen since 2015.

But, in a clear break with its Saudi partners, the UAE said in July that it was reducing its forces in the country, and fighting escalated between separatists and government forces on the ground in August. The UAE has since thrown its support behind the separatist movement.

That month, separatists said they'd taken control of the strategic port city of Aden after days of fighting with government forces.

A couple of weeks later, the Yemeni government accused the UAE of carrying out a series of airstrikes that killed dozens of its troops -- but the UAE said it was targeting terrorist militias.

Saudi-backed forces have since regained control of Aden and talks are under way to end the power struggle over the city, news agencies report.

As infighting has escalated in southern Yemen, ISIS has seized the opportunity to resurface there.

It claimed responsibility for a series of attacks in Aden in August, the first in more than a year and a worrying sign that the conflict is creating a vacuum for extremists to flourish.

US lawmakers have reacted with outrage to CNN's new findings.

One of them, Senator Elizabeth Warren, a frontrunner to become the Democratic Party's presidential candidate in 2020, said: "One report of US military equipment ending up in the hands of our enemies is troubling. Two reports is deeply disturbing."

Warren said she intends to pursue the findings with the Trump administration, adding: "The Defense and State Departments have not yet provided answers to questions I asked back in February and I intend to follow up."

Breaking arms agreements

Working with local journalists, CNN was able to film a number of US-made Mine-Resistant Ambush Protected (MRAP) vehicles that have been used by separatist militia groups led by the Southern Transitional Council (STC).

One was a BAE Caiman that was used by separatists in the fight for Shabwah, southern Yemen, against government forces in August.

It, like several other pieces of weaponry identified, can be traced back to a $2.5 billion arms sale contract between the US and the UAE in 2014.

Like all arms deals, this contract was bound by an end user agreement which certifies the recipient -- in this case the UAE -- as the final user of the weaponry. From this evidence, it is clear that this agreement has been broken.

In February, the UAE told CNN that there was no violation of end use requirements "in any manner."

Inside the vehicle was further evidence of its provenance. Its air conditioning system had a serial number from American company Real Time Laboratories, indicating the part was manufactured at its facility in Mississippi.

When asked whether it knew if its technology was ending up in the wrong hands in Yemen, Real Time Laboratories said they had supplied the product to BAE Systems in 2010 under a US government subcontract, but "cannot comment on what the US Government may have eventually done with this vehicle."

A spokeswoman for BAE Systems, which manufactured the Caiman MRAP vehicle, said the company complies with "all relevant export control laws and regulations in the countries in which we operate," and referred CNN to the Pentagon for further comment.

In response to new findings, Pentagon spokesperson Lt. Col. Carla Gleason said in September that the joint investigation by the State Department and Department of Defense into unauthorized weapons transfer in Yemen remains "ongoing."