The United States and Turkey signed an agreement last month to train and equip some Syrian rebels, but Washington still cannot convince Ankara to increase its commitment to the fight against the Islamic State.

The United States has been trying for months to convince Turkey to allow the coalition to fly manned airstrikes from the Incirlik Air Base, use Turkish troops on the ground in Syria to aid the airstrikes, and do more to restrict the flow of foreign fighters from Turkey into Syria. The Turkish government has been asking for two things in return: a limited "air exclusionary zone" to protect its troops inside Syria and a more concerted effort to oust Syrian President Bashar al-Assad.

Senate Foreign Relations Committee Chairman Bob Corker traveled to Turkey over the last congressional recess and met with President Recep Tayyip Erdogan. Corker told Bloomberg reporters last week that the administration's failure to secure Turkish commitment to the fight against ISIS was a huge problem.

"So now we're going to create this train-and-equip program, where we take people out of the country to other places and train them against ISIS," he said. "Which brings up the second piece, which is not heartening yet, and that is that in order to get Turkey into the conflict in a meaningful way, there has to be some agreement over how we're going to deal with Aleppo."

The moderate opposition forces in Aleppo are losing a three-front war there to Islamic State, the Assad regime, and the al-Nusra Front, an al-Qaida affiliate. The largest Western-backed group there, Harakat Hazm, disbanded Sunday after brutal defeats and a cut-off of weapons from the international community.

Retired Gen. John Allen, the Obama administration's special coordinator for the coalition to counter Islamic State, has been discussing with Turkey an air exclusionary zone in Northern Syria since last November. The zone would provide some protection for Western-backed moderate rebel fighters and Turkish troops on the ground. The White House has repeatedly shot down the idea in public and private, frustrating the Turks.

"The Turkish government is really not willing to come to the table unless the administration is at a minimum willing to commit to that occurring and they've not been willing to do that," said Corker.

Allen's spokesperson Emily Horne told me Monday that the U.S.-Turkey agreement signed Feb. 19 in Ankara to train and equip Syrian rebels was independent from negotiations about the other large issues, such as the use of Turkish military facilities and the role of Turkish troops on the ground.

"This was a specific agreement regarding a specific program," she said. "The program aims to strengthen moderate Syrian opposition forces to defend the Syrian people from attacks by ISIL and secure opposition-held territory; protect the United States, its friends and allies, and the Syrian people from the threats posed by terrorists in Syria; and promote the conditions for a negotiated settlement to the conflict in Syria."

Corker said the two issues were related because the Obama administration hasn't decided whether it will provide air cover for the Syrian rebels that it trains, arms, and implants back into the fight. If Turkey isn't more prominently involved on the ground in Syria, the U.S. plan to train and arm the Syrian rebels could fall apart, especially for those rebels near the Turkish border.

"You can't just send people out in the field. You've got to support them. You've got to logistically ensure that they have materials. You've got to make sure that they hopefully have close air support when they're doing the things that they're doing," Corker said. "The president hasn't agreed to do that yet. And it is a major sticking point."

The White House and the Erdogan government seem at a stalemate. "For the Turks, they want a strategy that involves fighting and getting rid of the Assad regime. But the U.S. doesn't have one," said Andrew Tabler, senior fellow at the Washington Institute for Near East Studies. The White House wants to prioritize the fight against Islamic State, and deal with Assad later, preferably with a political negotiation.

The lack of robust Turkish participation is also harming the current mission. For example, most U.S. airstrikes inside Syria currently fly out of Kuwait, raising the cost and duration of every sortie. The absence of spotters on the ground makes targeting less reliable.

Unless the White House makes some concessions to Ankara, Erdogan is unlikely to accede to the Obama administration's demands. And unless Turkey fully joins the fight against Islamic State in Syria, the coalition effort there is unlikely to succeed.

Josh Rogin is a Bloomberg View columnist who writes about national security and foreign affairs.