Saudi Arabia's critics in Congress say they may seek to block a new Obama administration proposal to sell the kingdom $1.15 billion in tanks, machine guns and other U.S. military equipment.

The proposed sale was announced this week by the Pentagon's Defense Security Cooperation Agency, which facilitates arms sales to foreign countries. Congress, on an annual summer break until next month, has a 30-day window to pass legislation blocking the sale after it reconvenes.

Previous efforts to restrict arm sales to Saudi Arabia have come up short, but lawmakers are increasingly concerned amid reports that the country's oil-rich Wahhabi rulers armed rebels in Syria's civil war that included jihadi groups and that the country is slaughtering civilians in a campaign against Shiite groups in Yemen.

"Saudi Arabia is an unreliable ally with a poor human rights record," says Sen. Rand Paul, R-Ky., in an emailed statement. "We should not rush to sell them advanced arms and promote an arms race in the Middle East. I will work with a bipartisan coalition to explore forcing a vote on blocking this sale."

Saudi Arabia recently has been a massive buyer of U.S. arms. In 2010, it received approval to buy up to $60 billion in fighter planes and military helicopters and late last year received rapid-fire permission to buy hundreds of air-defense missiles for $5.4 billion in September, four military ships for $11.25 billion in October and 22,000 smart and general purpose bombs for $1.29 billion in November.

"This proposed sale will contribute to the foreign policy and national security of the United States by helping to improve the security of a strategic regional partner which has been and continues to be a leading contributor of political stability and economic progress in the Middle East," the Defense Security Cooperation Agency said in its announcement of the latest sale proposal.

But members of Congress alarmed in particular by the Saudi-led intervention in Yemen that began in March 2015 aren't willing to allow the latest deal to proceed unchallenged.

A natural Paul ally in questioning the new deal is Sen. Chris Murphy, D-Conn. – with whom he's sponsoring legislation that would require the president to certify that any arms sold to Saudi Arabia are not being given to terrorists or used recklessly to kill civilians.

"I believe that U.S. weapons sales should first and foremost advance American security interests and will take a close look to make sure that's the case here. The Saudis have largely backed away from the military fight against ISIS, and I'd like to see them commit to rejoin that fight as part of major new military sales," Murphy said in a statement, referring to the ongoing campaign against the Islamic State group. "I also continue to have concerns over the high rate of civilian casualties in Saudi Arabia's Yemen operations."

The latest proposed sale would include up to 153 Abrams tanks, 20 Hercules armored vehicles, 153 M2 .50 caliber machine guns and 266 7.62mm M240 machine guns.

The Saudi-led intervention in Yemen, nominally on behalf of its internationally recognized government, regularly is reported to have killed civilians, despite the U.S. government reportedly helping with target selection to limit such deaths. On Tuesday, for example, news outlets reported at least seven civilians died in Yemen's capital Sanaa, most of them in a potato chip factory.

The Shiite Houthi movement and various Yemeni armed forces loyal to the country's longtime U.S.-allied former president, Ali Abdullah Saleh, have put up a fight against the Saudis, who say 20 of the tanks they want to purchase would replace damaged vehicles.

With resolution of the Yemeni conflict appearing distant, members of the U.S. House of Representatives also are objecting to the proposed sale.

"I believe the Saudi military's operational conduct in Yemen and the killing of civilians with U.S.-made weapons have harmed our national security interests, and I will continue to oppose any arms sale that contributes to its operations in that arena," says Rep. Ted Lieu, D-Calif. "This approved sale deserves to be scrutinized by Congress rather than rubber-stamped during the summer recess."

Rep. Jim McGovern, D-Mass., offered similar reasoning.

"Saudi Arabia's dangerous and reckless use of cluster munitions and other weapons has already harmed and killed countless innocent civilians. The last thing the United States should do is sell them more than a billion dollars' worth of additional arms and military equipment," he says.

"As a world leader, America has a solemn responsibility to stand up for human rights and help protect the lives of innocent civilians," McGovern adds. "This arms sale sends the wrong signal to Saudi Arabia and undercuts the strong concerns expressed by the U.S., the United Nations and other countries about the Saudis' failure to take the necessary steps to avoid civilian casualties."

Moral indignation alone, however, may be insufficient to block the sale – for which Congress would have to pass legislation at an unusually fast pace.

Similar concerns about arms sales were raised in June after the United Nations reported that in 2015 the Saudi-led coalition killed 510 children and wounded 667 others in Yemen. (The U.N. subsequently backed off labeling the Saudi-led coalition a violator of children's rights in combat after the Saudis threatened to withdraw U.N. funding.)

Shortly after the U.N. report was published, the U.S. House of Representatives considered banning the sale of cluster munitions to Saudi Arabia. The weapons are a subset of air-delivered bombs that spread explosives over a wide area, maximizing carnage and sometimes leaving unexploded devices behind.

The amendment offered by Rep. John Conyers, D-Mich., to ban the sale of cluster munitions to Saudi Arabia was defeated 216-204 . Rep. Rodney Frelinghuysen, R-N.J., opposing the amendment, said "the Department of Defense strongly opposes this amendment" and that "they advise us that it would stigmatize cluster munitions, which are legitimate weapons with clear military utility."

It's possible an effort to block the current arms sale proposal would have less support than the legislation on cluster munitions, which are more closely tied to alleged human rights abuses. But perhaps that surprisingly close vote would embolden fence-sitters to join opponents.

The Saudi embassy did not respond to a request for comment.

If there is momentum to block the sales to Saudi Arabia, which in recent years has been one of the biggest foreign customers for American arms dealers, there's sure to be a counter-fight led by hawkish members of both political parties, who are likely to stress Saudi opposition to Iran-friendly groups in the region.