WASHINGTON -- Over the past six months, the Obama administration has quietly embroiled the U.S. in a Middle Eastern war that has left more than 20 million people in need of humanitarian aid and killed at least 5,000 .

U.S.-backed Sunni Arab nations, led by Saudi Arabia, have bombed weddings , left families to starve and looked the other way as an al Qaeda affiliate has used the confusion to seize significant territory. Civilian casualties are growing daily.

The White House won't admit that the U.S. is even "in" Yemen. But it's refueling the planes bombing the country and providing intelligence to the Sunni states running the Yemen campaign. Now lawmakers, dissenters within the administration and human rights activists are ramping up their criticisms of the Obama policy. They argue that the U.S. is callously backing the Saudi-led coalition -- in part to reassure America's Sunni allies in the wake of the nuclear deal with Shiite Iran -- without concern for the consequences. By supporting the Saudi effort, they say, President Barack Obama risks empowering al Qaeda and implicating the U.S. and its allies in war crimes -- not to mention further tarnishing America's already damaged image in the Middle East.

"It's time for Congress to ask some serious questions about whether the United States current participation in this civil war is advancing our nation's national security interests," Sen. Christopher Murphy (D-Conn.) said in a statement Thursday.

Murphy and other lawmakers plan to increase their pressure on the administration this week. In his statement, Murphy suggested they may go as far as blocking future arms sales to Saudi Arabia until their humanitarian concerns are addressed -- which means this largely overlooked conflict could soon endanger a cornerstone of the Obama administration's policy in the Middle East.

A new stage of an old war

Under Obama, the U.S. has consistently conducted drone strikes in Yemen since 2009 and treated that country's government as a "key partner ." (President George W. Bush approved one strike there in 2002 .) That means Washington is already to blame for the deaths of scores of Yemeni civilians . But it expanded its role in the country in March, when Saudi Arabia made clear to U.S. officials that it wanted to launch a military campaign against the Houthis, a rebel group that pushed the Saudi-friendly Yemeni government out of Yemen's capital last year. The White House approved American support for its Sunni Arab allies in the Gulf Cooperation Council, conscious that the Sunni-led powers were worried about nuclear negotiations then ongoing with Iran. The U.S. provided intelligence for the initial strikes in Yemen by the Saudis and their coalition, the Wall Street Journal revealed in April.

Since then, the U.S. has only gotten more involved. U.S. forces have established a joint planning cell with Saudi Arabia to monitor Yemen and provide the Saudi-led coalition with intelligence on potential strike targets, Commander Kyle Raines of U.S. Central Command told The Huffington Post. The U.S. also provides aerial refueling for planes flying with the coalition, he said.

Raines noted that the U.S. refuels coalition planes outside of Yemeni airspace. That caveat is important -- it dovetails with rhetoric from other administration officials, who are keen to maintain that Obama has not actually sent U.S. forces into Yemen or made targeting calls .