American military intelligence chief has said that Russian President Vladimir Putin can no longer achive the goal with which he began the invasion of Ukraine.

Putin ordered the invasion of Ukraine in February — dubbed as a special military operation. The West believes that it was aimed at overthrowing the West-backed Ukrainian government led by President Volodymyr Zelenskyy. Russia's setbacks and stretched resources in Ukraine show its forces are incapable of achieving that goal, said the chief of US Defence Intelligence Agency (DIA) on Friday.

"We're coming to a point right now where I think Putin is going to have to revise what his objectives are for this operation. It's pretty clear right now that he's not going to be able to do what he initially intended to do," said DIA chief Lt. Gen. Scott Berrier.

At the beginning of the invasion, the Russians believed they would defeat Ukraine within days and would capture the capital Kyiv. However, the Ukrainians thwarted the Russian offensive at Kyiv, following which Russian forces were redeployed from around Kyiv and elsewhere in Northern Ukraine to the country's east.?

This month, Ukrainians mounted a counter-offensive in Easter Ukraine, forcing Russians to retreat from key areas and retaking over 3,000 square kilometres of land held by Russians for months. This has further affected the Russian war efforts.

"The Russians planned for an occupation, not necessarily an invasion, and that has set them back," said Berrier, citing Putin's reluctance so far to fully mobilise Russian forces to get more manpower into the fight.

US President Joe Biden and other US officials have taken care not to call Russia's latest retreat a Ukrainian victory or turning point in the war, and analysts caution it's impossible to assess what may lie ahead.

"He's coming to a decision point. What that decision will be we don't know. But that will largely drive how long this conflict lasts," said Berrier of Putin.

Berrier spoke at a panel with other senior officials at the intelligence community's Intelligence and National Security Summit at National Harbour in Maryland outside Washington DC.

Asked about concerns that Putin could unleash weapons of mass destruction [WMDs] if he's thwarted on the battlefield by US- and NATO-backed Ukrainian forces, US Central Intelligence Agency (CIA) Deputy Director David Cohen said, "I don't think we should underestimate Putin's adherence to his original agenda, which was to control Ukraine. I don't think we've seen any reason to believe he has moved off that."

Nor should the United States underestimate Putin's "risk appetite", said Cohen. Putin and his officials early in the war made allusions to Russia's nuclear arsenal and to massive retaliation in warning NATO not to get involved in the conflict.

"That being said, we have not seen concrete evidence of planning for the use of WMD," Cohen said. The more likely form of any Russian retaliation against the United States would be more attempts at interfering with the US political system, other security and intelligence officials said.

Separately, in the Shanghai Cooperation Organisation (SCO) in Uzbekistan on Friday, Putin vowed to press the attack on Ukraine and warned that Moscow could ramp up its strikes on the country's infrastructure if Ukrainian forces target facilities in Russia. The conference included the leaders of China, India, Turkey and several other countries.

Putin said the "liberation" of Ukraine's entire eastern Donbas region was Russia's main military goal and that he saw no need to revise it. He added, "We aren't in a rush."

However, Russian forces in Eastern Ukraine are not in a good position, according to latest reports.

The think tank Institute for the Study of War (ISW) reported that Ukrainian forces captured all of Kupyansk City on Friday, adding that Ukrainians are pressing with their counter-offensive to which Russia has responded with limited ground assaults.

"Russian forces will likely struggle to hold positions in eastern Kharkiv Oblast and in northern Luhansk Oblast as Ukrainian forces establish more positions on the east bank of the Oskil River...Russian forces conducted limited ground attacks across the Eastern Axis and continued routine fire along the line of contact on September 16," said ISW.

The British Ministry of Defence on Saturday said in an intelligence update on Ukraine that Ukrainian offensive continues in Eastern Ukraine where Russia would attemp a "stubborn" defence but its capacity to do so are uncertain.

It reported, "Ukraine continues offensive operations in the north-east of the country while Russian forces have established a defensive line between the Oskil River and the town of Svatove.

"Russia likely sees maintaining control of this zone as important because it is transited by one of the few main resupply routes Russia still controls from the Belgorod region of Russia.

"Russia will likely attempt to conduct a stubborn defence of this area, but it is unclear whether Russia’s front line forces have sufficient reserves or adequate morale to withstand another concerted Ukrainian assault."

The region is significant to Russia as the military line being established by Ukraine there "sits along the border of Luhansk Oblast, part of the Donbas". 'Liberating' Donbas is the main stated Russian objective in the region.

"Any substantial loss of territory in Luhansk will unambiguously undermine Russia’s strategy [of 'liberating' Donbas]," said the British update.

(With AP inputs)