The U.S. knows the Saudi government has employed cluster bombs in its ongoing war against Shiite Muslim rebels in neighboring Yemen, but has done little if anything to stop the use of the indiscriminate and deadly weapons during what has become a human rights catastrophe in one of the Arab world's poorest countries.

With watchdog groups warning of war crimes and attacks striking civilians in Yemen, the Pentagon declined to comment publicly on whether it has discussed cluster bombs with Saudi Arabia or encouraged its military to cease using them, deferring all such questions to the State Department. But a Pentagon official, speaking on the condition of anonymity, tells U.S. News "the U.S. is aware that Saudi Arabia has used cluster munitions in Yemen."

Deferrals by the Pentagon on the topic are nothing new, though the use of the weapons by the Saudis – some of which were reportedly supplied by the U.S. – appears to be only a recent tactic. Former spokesman Army Col. Steve Warren told reporters in May the Defense Department was looking into claims the Saudis were using cluster munitions and called on all sides to "comply with international humanitarian law, including the obligation to take all feasible measures to minimize harm to civilians." Warren's successor, Navy Capt. Jeff Davis, was asked about similar reports in July and did not at that time have any new information.

Multiple groups are fighting in Yemen, but the heart of the conflict lies between forces loyal to U.S.- and Saudi-backed President Abed Rabbo Mansour Hadi, who fled from Yemen to Saudi Arabia earlier this year. They're fighting against Shiite Houthi rebels aligned with, if not directly backed by, chief Saudi rival Iran. Deposed Yemeni President Ali Abdullah Saleh has also re-emerged and allied himself with the Houthis.

The U.S. supports the Saudi-led coalition of Arab nations battling the Houthis with an operations center in Saudi Arabia and another in Bahrain. Through them, the American military provides intelligence and logistics support as well as air tankers to help refuel the coalition's jets.

That assistance, however, doesn't grant the U.S. much sway over the way Saudi Arabia is waging its war.

"This is quite new for Saudi Arabia to be so assertive in their foreign policy and the use of their military, which is precisely why the Pentagon is bending over backward [for them]," says Charles Schmitz, a Towson University professor and expert on Yemen. "They want to reassure the Saudis that the U.S. is still on their side, so they're letting them do whatever they want."

Clashes have taken place throughout Yemen, but have been focused on recent weeks in and around the key port city of Aden, where Saudi-led forces established a beachhead against the Houthi stronghold earlier this month and have slowly expanded outward.

The conflict has grown increasingly deadly, and the deployment of cluster bombs has only added to the carnage. Almost 4,000 people have been killed, with 19,000 injured and more than a million displaced from their homes, according to accountings by the Red Cross and other organizations.

The U.S. and Saudi Arabia are among 80 countries that have not signed The Convention on Cluster Munitions, a treaty banning the use of such weapons, according to the Cluster Munition Coalition. The Defense Department also has deemed the bombs "legitimate weapons with clear military utility."

Indeed, from a pure military perspective, a cluster bomb is ideal. The ordnance – which breaks apart in flight to disperse multiple, smaller explosives – is an excellent "area denial weapon" in military-speak, with its ability to cause massive destruction over wide swathes of territory while using relatively few military personnel. In Yemen, a largely arid country that shares a long border with Saudi Arabia, such weapons can be used to great effect.

But cluster bombs are also very difficult to control and extraordinarily dangerous to noncombatants. The explosives disperse more widely than precision-guided weapons and may not detonate on impact, making them potentially deadly long after combatants have left a battlefield.

"These weapons should never be used under any circumstances," Human Rights Watch arms director Steve Goose said in May, when his organization released a report alleging the use of cluster bombs in Yemen. "Saudi Arabia and other coalition members – and the supplier, the U.S. – are flouting the global standard that rejects cluster munitions because of their long-term threat to civilians."

However, since the U.S. is not leading the current war in Yemen – and since it hasn't sworn off such weapons itself – it is no position morally or militarily to dictate the actions of a partner like Saudi Arabia.

"Actual U.S. strategy in the Middle East right now is to try and get allies and proxies to take the lead on actual fighting, a variant of the 'lead from behind' approach [taken] in Libya," says Chris Harmer, a 20-year Navy officer now with the Institute for the Study of War. "It is simply not possible for the U.S. to tell allies and proxies who are doing the fighting what weapons to use. If the U.S. wants to minimize the use of cluster munitions against terrorist-affiliated groups such as the Houthis in Yemen, then the U.S. needs to take the lead in the fight.

"Absent a willingness to do so, the U.S. has no standing to tell its allies how to conduct the fight."

Like most in the Middle East, the Saudi military has dedicated itself largely to internal security, not external operations. That has changed during the tenure of President Barack Obama, who has encouraged allies in the region – the beneficiaries of expensive U.S.-developed military equipment – to fight neighborhood wars for themselves instead of expecting American intervention.

One of the main exceptions aside from Yemen was when Saudi Arabia quietly launched a ground campaign into neighboring Bahrain during the 2011 Arab Spring uprisings in an effort to rescue the ruling government's fellow Sunni Muslims from bubbling discontent among the Shiite majority there. Saudi-backed government forces were accused of heavy-handedness, though those reports were largely drowned out by larger uprisings elsewhere at the time. Many believe the harsh response was meant also to send a signal to Iran, believed to have been involved in stirring the uprising in Bahrain.

In Yemen, reports from the ground indicate the violence has reached new heights in recent weeks, following the collapse of a humanitarian cease-fire in late July after less than a day.

"The situation is getting worse again," Pitambar Aryal, the acting country representative for the International Federation of Red Cross and Red Crescent Societies, tells U.S. News by Skype from Sana'a, Yemen's capital. Workers with his group tell him "it's getting very difficult in managing bodies and injuries" amid heavy airstrikes and ground fighting in the country's predominantly Shiite south.

Red Crescent ambulances have fallen under direct attack and four volunteers have died, Aryal says, though it's unclear who initiated the attacks. A branch office building in the southern region of Taiz also was attacked.

Supply lines additionally have been cut off, accounting for massive shortages in food, water and medicine, Aryal says. There are up to two-day delays at gas stations, and the scarcity of fuel means power is only available for roughly an hour every three to four days.

Peter Maurer, president of the International Committee of the Red Cross, visited Yemen this month and called the situation there "nothing short of catastrophic."

"Every family in Yemen has been affected by this conflict," he said. "The people are facing immense hardship. And it is getting worse by the day. The world needs to wake up to what is going on."

Amnesty International released a report on Monday highlighting the unnecessary deaths of Yemeni civilians in air and ground attacks, saying these noncombatants are bearing the brunt of the raging conflict. Houthi fighters and forces loyal to the government have killed civilians in small-arms crossfires and with the use of land mines, the report said, while the death toll also is climbing due to what the advocacy group calls the Saudi-led coalition's "unlawful airstrikes which failed to distinguish between military targets and civilian objects in [Houthi]-controlled areas."

Among the thousands of airstrikes that have taken place since March, Amnesty investigated eight that it said accounted for at least 141 civilian deaths and more than a hundred injuries.

Reports Saudi Arabia targeted and destroyed the Houthi group's symbolic shrine also contribute to concerns its air campaign is designed to punish the Yemenis, Schmitz says, not simply to operate militarily against them.