The United States' top general has warned Ukraine has just six weeks left before changing weather hampers its counteroffensive, even as Kyiv is signaling it could fight on into the winter.

"There is still a reasonable amount of time, about 30 to 45 days' worth of fighting weather left," Chairman of the Joint Chiefs of Staff Gen. Mark Milley told the BBC on Sunday. After that, mud and rain would likely have an impact on battlefield maneuverability, he said.

In keeping with the "glass half full" messaging coming from Biden administration officials of late, Milley said the counteroffensive had achieved "very steady progress" since it began in early June.

"The Ukrainians aren't done, this battle is not done, and they haven't finished the fighting part of what they are trying to accomplish. It's too early to say how this is going to end," he said.

Reports on Sunday suggested only incremental gains around one of Ukraine's main areas of attack, near the village of Robotyne in Zaporizhzhia region, which lies on the way to the strategic town of Tokmak.

An unofficial Telegram channel belonging to Ukraine's 46th brigade, which has proved a reliable source of information, said troops had advanced to the east of a neighboring village, but cautioned Russian forces still held higher ground nearby, giving them an advantage.

Meanwhile, an online update from the southern 'Tavria' command said: "we continue to make small advances in the area of Robotyne. About 1.5 square kilometers of Ukrainian territory have been liberated," though it did not say how long a period of time it was referring to.

'The counteroffensive will continue'

Asked about his forces' rate of advance on Friday, President Volodymyr Zelensky insisted Ukraine still had the initiative, but he urged a conference audience in Kyiv not to see the counteroffensive like a feature film that was all done in 90 minutes.

One of Zelensky's senior security officials, speaking at the same event the following day, indicated Ukraine's forces might be set to keep attacking well into the coming winter.

Kyrylo Budanov, head of military intelligence, acknowledged the counteroffensive was moving more slowly than he would like. Russia's defensive lines were well-planned, he said, and heavily laid with mines, which made the situation on the battlefield "complicated."

But even though cold weather was a reality the military cannot ignore, "hostilities will continue, the counteroffensive will continue," he said.

Earlier in the week, retired US General David Petraeus appeared to offer a similar prognosis at a separate conference in Kyiv, saying, "this will go through the winter."

Comparisons with Ukraine's first big counteroffensive, in 2022, also suggest a longer timeframe could be possible.

Russian forces were rolled eastwards out of Kharkiv region exactly a year ago, culminating in the recapture of the town of Lyman by Ukrainian forces around September 30.

Ukraine's counteroffensive in the south then ran for a further six weeks, only ending around November 10 with the liberation of Kherson.

It remains a topic of debate among analysts and policymakers whether Ukraine should have kept up the momentum, or whether western allies had failed to supply enough weapons and ammunition to make that possible.

There are also fears expressed by Kyiv's supporters that any halt in offensive operations over the coming winter would only serve to give Russia the chance to further strengthen its defenses.