Afghan President Ashraf Ghani last month unveiled a plan to open talks to end the 16-year-old war, offering to negotiate with the Taliban without any preconditions.

So far the group's response to the offer has been muted, which analysts said reflects debate among Taliban leaders over the merits of engaging with an administration it has long viewed as illegitimate.

But US officials including Defense Secretary Jim Mattis said this week that some Taliban elements are open to talking with the Afghan government.

General John Nicholson, who leads US and NATO forces in Afghanistan, said the Taliban have taken heavy casualties since US President Donald Trump authorised ramped-up air operations last year, pointing to increasingly effective Afghan commando and regular Afghan army units.

"In the Taliban's mind, they see what is coming and these capabilities are only going to get greater," Nicholson told reporters accompanying Mattis on a visit to Bagram Airfield, America's largest air base in Afghanistan that is located north of Kabul.

"So this really is probably their best time to attempt a negotiation, because it's only going to get worse for them," he added, as both sides prepare for the start of what is expected to be an intense spring fighting season.

Ghani's peace plan includes eventually recognising the Taliban as a political party. In return, the Taliban would need to recognise the Kabul government and constitution -- a perennial sticking point in past attempts to open talks.

Despite Nicholson's tough talk, US data show the Taliban is far from being driven off the battlefield.

In October, insurgents controlled or influenced nearly half of Afghanistan's districts -- double the percentage in 2015, the US government's office of the Special Inspector General for Afghanistan Reconstruction said in January.

Over the same period, the watchdog said, the number of districts under Afghan government control or influence fell to its lowest level since December 2015.

- Focus on Kabul -

"My perception of what is going on inside the Taliban is they are tired of this war as well, they'd like to return home, they'd like to rejoin society and, just like the people of this country, would like to see the end of this war as would all of us," Nicholson said.

He added that there are "many Taliban who could see a way to work within this framework" but cautioned there would always be those that will never reconcile.

"It's encouraging that these offers are on the table and we would appear to be at a point where they could start having a conversation about this," he said.

Aside from military pressure, Nicholson said it is important that diplomatic pressure is strong on "those who externally enable the insurgency" and he credited the role that religious pressure from other Islamic countries is playing.

The four-star general also underscored the need to strengthen security in Kabul, where a string of devastating attacks in recent months has slain hundreds of civilians.

"Kabul is our main effort right now, to harden Kabul, to protect the people of Kabul and the international community that are here because of the strategic impact that has and the importance to the campaign," he said.