The New York Times reports:

The United States has been passing messages indirectly in recent days to the rebel groups that led the lightning offensive to topple President Bashar al-Assad of Syria, officials said.

Now that the groups have taken Damascus and succeeded in pushing Mr. Assad out of power, President Biden and his top advisers are debating the extent to which they should engage directly with them going forward, the officials said.

The United States started passing messages to the groups through the Turkish government, which has ties to them, after they launched their major offensive last week and before they took the Syrian capital over the weekend, according to U.S. and Turkish officials. One of the officials said the initial messages were meant to tell the groups “what not to do.”

One of the messages that the United States passed through the Turkish government was that the rebel groups should not allow militants with the Islamic State to participate in their offensive against the Assad government.

The groups responded through the Turks with assurances that they had no intention of allowing the Islamic State to be part of their movement, according to U.S. and Turkish officials briefed on the messages.

The messages have been passed to the Turks using diplomatic, intelligence and military channels.

The officials spoke on condition of anonymity to discuss sensitive intelligence and diplomatic matters.

In his public remarks from the White House on Sunday, Mr. Biden said the U.S. would not allow the Islamic State to take advantage of any power vacuum following Mr. Assad’s fall.

The broader debate within the Biden administration over engaging more directly with the groups centers on their past behavior and connections to extremists. It also encompasses ongoing assessments by U.S. intelligence analysts and administration policymakers of whether the groups have, or are prepared to, substantially change their ways to address the concerns of the United States and its allies in the region about their terrorist affiliations.

The rebel groups have united under the leadership of Hayat Tahrir al-Sham, or H.T.S., which the United States branded a foreign terrorist organization during the Trump administration.

H.T.S. is a former Al Qaeda affiliate, but it broke with the older group years ago and came to dominate the last stronghold of Syria’s opposition.

Once seen as one of the rebellion’s most powerful extremist factions, the group later tried to play down its radical aspects and focused on building something like a civilian government — albeit an authoritarian and extremist one — in its patch of territory.

U.S. intelligence agencies and top officials in the Biden administration are still in the process of evaluating H.T.S. and its leader, Abu Mohammad al-Jolani, who is eager for legitimacy and has mounted what one senior U.S. official characterized as a “charm offensive” aimed at allaying concerns about their intentions and past affiliations.

The way one senior U.S. official put it, “A charm offensive might mean that people are turning over a new leaf and they think differently than they used to so you should hear them out. On the other hand, you should be cautious because charm offensives can sometimes be misleading.”

The official continued, “We have to think about it. We have to watch their behavior and we need to do some indirect messaging and see what comes of that.”

U.S. officials said the Biden administration is allowed to talk to H.T.S. and its leader even though they are on the terrorist list, but the administration cannot provide them with material support.

While the Biden administration has so far stopped short of directly talking to H.T.S., it has been working closely and directly with the U.S. military’s main counterterrorism partner in Syria, a Kurdish-led militia known as the Syrian Democratic Forces, or S.D.F.

According to U.S. officials, the administration encouraged and provided intelligence support to the S.D.F. for operations by the group to take control of Syrian territory in eastern Syria, including the cities of Deir al Zour and Abu Kamal.

The officials said the operations were meant to ensure that the Islamic State couldn’t take advantage of the situation and seize the areas as Syrian government forces withdrew.

According to a U.S. official, the message that the United States sent to the S.D.F. was, “If the regime vacates territory, it’s going to go to somebody, so you should fill the vacuum yourself rather than let ISIS fill into that vacuum.”