U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov walk into their meeting room in Geneva on Sept. 9, 2016. (Kevin Lamarque/AP)

The United States and Russia struggled Friday to make headway in talks over a possible Syria peace deal, saying that an agreement was possible but expressing little optimism it was imminent.

Meanwhile, the U.N. envoy to the Syrian crisis, Staffan de Mistura, described the worsening humanitarian situation inside Syria and urged the parties to reach a deal to ease the fighting and allow the flow of desperately needed relief aid.

In broad terms, the proposed pact calls for a “pause” in the fighting between the U.S.-backed Syrian opposition and forces of President Bashar al-Assad and his Russian and Iranian allies — particularly around the besieged city of Aleppo — to allow safe and sustained delivery of humanitarian aid.

That would followed by a more wide-ranging cease-fire, along the lines of a truce agreed last February that lasted only several weeks.

Syrian opposition leaders have refused to return to U.N.-brokered political talks until the fighting stops — particularly Syrian and Russian bombing of civilian areas. On a separate front, a U.S.-led coalition is also conducting airstrikes against Islamic State strongholds in Syria.

“We can’t guarantee in any way at this point that we are on the cusp of finishing,” a senior administration official told reporters who flew to Geneva with Secretary of State John F. Kerry. “That said, I think if we didn’t think that . . . it remained a possibility of getting this done, we wouldn’t be going back to Geneva.”

“We’re going to try, but our patience is not infinite,” a second senior official said. “We’re not going to just keep going . . . if we don’t reach a conclusion relatively soon.”

The officials spoke on condition of anonymity to discuss the talks.

An early morning meeting between Kerry and Russian Foreign Minister Sergei Lavrov — their third in less than two weeks — broke after little more than an hour. Both envoys retreated temporarily to their respective suites in a lakefront hotel to consult with their capitals and address other pressing matters, such as the latest new nuclear test by North Korea.

Ninety minutes later, the two diplomats, trailed by briefcase-toting aides, strode back across the hotel lobby into a basement meeting room, and closed the door. That pattern continued throughout the day, with frequent pauses for consultations at home.

As the day-long talks moved into the evening, the two sides were said to be “making progress.” But “we are not in a position right now to say whether or not a final deal can be reached,” a senior State Department official said, speaking on condition of anonymity to discuss the negotiations. Both Kerry and Lavrov indicated they would not continue the current round of discussions and will leave Geneva Friday night, regardless of whether an agreement is concluded.

Final terms of any agreement must be approved by President Obama and others with equities in the deal, including some in the White House and the Pentagon, where there is deep skepticism about Russian intentions.

The proposed agreement — first authorized by Obama and presented by Kerry to Russian President Vladimir Putin in mid-July — has been refined several times in response to demands of various parties and changing circumstances in Syria. Kerry delivered the most recent U.S. version to Lavrov when they met last weekend in China, where Obama also met with Putin on the sidelines of a Group of 20 meeting.

Kerry decided to travel Thursday afternoon to Geneva after receiving a written response from Moscow.

“The opposition tell us they want us to reach a deal with the Russians if, in fact, it would stop some of the worst forms of violence against the Syrian people. So they want us to continue,” the second U.S. official said. “If we reach a deal, then that’s great. And if we don’t, we’re not going to go on forever for the sake of pursuing a deal.”

Russia has insisted it is targeting only designated terrorist groups that were not party to the original cease-fire, in particular the Front for the Conquest of Syria, or Jabhat Fatah al-Sham. The group, formerly known as Jabhat al-Nusra, recently announced it had broken with al-Qaeda.

But Syrian rebels, and their Western and Arab backers, claim that Russian air attacks have hit anti-Assad forces. Activists also have accused Syria of carrying out bombing raids using chlorine weapons, leaving hundreds of people stricken.

There are also questions of how to withdraw forces from access roads so that aid can be delivered, how to monitor compliance of both sides, and the sequence of withdrawals.

Once a sustained cease-fire and the rest of the agreement is in place, the administration has said it is willing to coordinate targeting and airstrikes against the Front and the Islamic State.

Some in the administration have argued that the Russians are only trying to run out the clock until the end of the Obama administration in January, in hopes of a better deal from his successor. Others insist that Moscow is aware that there is a better-than-even chance it could get a worse deal, particularly with the election of Hillary Clinton.

Asked whether the Russians were “playing for time” and dragging out the talks while helping Assad’s forces gain more territory on the ground, the second U.S. official said that “if we get to a point where we think they’re just playing for time, that’s probably when you’ll see us go in a different direction.”