NATO’s secretary general says Vladimir Putin’s invasion has not gone to plan and Ukraine “can win this war” as new intelligence indicated Russia may have lost one third of the force it sent to the country.

Speaking after a meeting of NATO foreign ministers, Jens Stoltenberg said Russia’s military was not achieving “its strategic objectives”.

“They failed to take Kyiv, they are pulling back from around Kharkiv, their major offensive in Donbas has stalled,” said Mr Stoltenberg.

“Ukraine can win this war.”

Ukrainian’s presidential adviser Mykhaylo Podolyak said his country could win the war this year but would need key ingredients in a “recipe for victory”.

Mr Podolyak tweeted that Ukraine needed heavy weapons from the West and a “real oil embargo” on Russia.

“Ukraine is not interested in a protracted war with Russia, same as the whole world.

“We can’t beat month this month (sic), but we can do it this year.

“The recipe for victory is simple: real oil embargo + tanks, aircraft and artillery. Let’s end it together,” he tweets in English.”

It comes as Russia faces further political setbacks after Finland on Sunday formally announced its desire to join NATO and Sweden also stated it supported joining the alliance.

Meanwhile Britain’s Ministry of Defence (MoD) said Russia may have lost one-third of its invasion force as its campaign in the east of Ukraine “lost momentum” and was now “significantly behind schedule”.

Russia’s Donbas offensive was unlikely to make rapid advances during the coming 30 days, the MoD assessment said.

The Russians switched the focus of their offensive to the eastern region of the Donbas, which was already part-held by pro-Moscow separatists, after their advance on Kyiv was driven back.

However the MoD said despite small-scale initial advances, they had failed to make any substantial territorial gains over the past month while suffering “consistently high levels of attrition”.

It said the offensive was being further hampered by the loss of “critical enablers”, such as bridging equipment and intelligence, surveillance and reconnaissance drones.

“Russia has now likely suffered losses of one-third of the ground combat force it committed in February,” it said.

“Russian forces are increasingly constrained by degraded enabling capabilities, continued low morale and reduced combat effectiveness.

“Many of these capabilities cannot be quickly replaced or reconstituted and are likely to continue to hinder Russian operations in Ukraine.”

The Finnish Parliament is expected to endorse its decision in coming days, to be followed by the submission of a formal membership application next week.

Mr Stoltenberg said if the two Nordic nations did apply it would be a “historic moment” for the alliance.

“Their membership in NATO would increase our shared security, demonstrate that NATO’s door is open, and that aggression does not pay,” he said.

But Turkey’s foreign minister has named its conditions to supporting Sweden and Finland’s admission to NATO, including to stop supporting terrorist groups in their countries.

Turkey’s foreign Minister Mevlut Cavusoglu said he met his Swedish and Finnish counterparts and all were seeking to address Turkey’s concerns.

He added that Turkey was not threatening anybody or seeking leverage but speaking out especially about Sweden’s support for the PKK Kurdish militant group, deemed a terrorist group by Turkey, the European Union and the United States.

Any decision on NATO enlargement requires unanimous approval by all 30 member states.

Battle for Donbas

The front lines in Ukraine have shifted again as Russia made some advances in the fiercely contested eastern Donbas region and Ukraine’s military waged a counter offensive near the strategic Russian-held city of Izium.

Near the north-eastern city of Kharkiv, where Ukrainian forces have been on the attack since early this month, commanders said they believed Russia had been withdrawing troops to reinforce positions around Izium to the south.

Ukraine has scored a series of successes since Russia invaded on February 24, forcing Russia’s commanders to abandon an advance on the capital Kyiv and then making rapid gains to drive them from Kharkiv, Ukraine’s second-biggest city.

Since mid-April, Russian forces have focused much of their firepower on trying to capture two provinces known as the Donbas after failing to take Kyiv.

Keeping up pressure on Izium and Russian supply lines will make it harder for Moscow to encircle battle-hardened Ukrainian troops on the eastern front in the Donbas.

Izium straddles the Donets river, about 120 kilometres from Kharkiv on the main highway heading south-east.

“The hottest spot remains the Izium direction,” regional governor Oleh Sinegubov said in comments aired on social media.

“Our armed forces have switched to a counter offensive there. The enemy is retreating on some fronts.”

Russia said on Sunday it had pummelled Ukrainian positions in the east with missiles, targeting command centres and arsenals as its forces seek to encircle Ukrainian units in the battle for Donbas.

But Ukraine’s military also acknowledged setbacks in an update on Sunday morning: “Despite losses, Russian forces continue to advance in the Lyman, Sievierodonetsk, Avdiivka and Kurakhiv areas in the broader Donbas region.”

-with AAP