Ukraine and Russia are fighting a "First World War-like" battle for the city of Bakhmut as casualties pile up on both sides, western officials have said.
But Britain said on Tuesday it was "not practical" to supply fighter jets despite the assessment of western analysts that the war is entering a stalemate.
Ukraine is pushing Nato members to supply their jets after US President Joe Biden ruled out sending F-16s.
Kyiv and Moscow are believed to be planning spring offensives as the war's anniversary approaches.
The eastern city of Bakhmut has been the site of heavy fighting as Ukraine awaits the arrival of western tanks.
"Both sides are suffering really heavy casualties, particularly around Bakhmut. It’s a devastating, First World War-like operating environment," one western official said.
The official said the race was on for which side could maintain its supply of weapons and build up its offensive capability.
"Even though on paper the Russians have greater mass, what we’ve seen is that they haven’t been able to employ that successfully on the battlefield," they said.
"The Ukrainians have an advantage from superior western military equipment that’s being provided.
"There’s some poor decision-making on the Russian side and there’s some pretty agile thinking happening on the Ukrainian side.
"When you stack up all of these positives and negatives, you end up in some sort of grinding conflict, which at the moment looks like it continues through 2023 unless something else significant changes."
Lithuania, a Nato member that was the first former Soviet republic to declare independence on the collapse of the Soviet Union, warned that the alliance must be prepared to send fighter jets and long range missiles.
After months of haggling, western allies last week agreed to send the tanks.
Ukraine will receive 120 to 140 of them in a "first wave" of deliveries from a coalition of 12 countries, Foreign Minister Dmytro Kuleba said on Tuesday.
Nato partners now must cross yet another red line and send fighter jets and long-range missiles to Ukraine, Lithuanian President Gitanas Nauseda said.
The UK said training Ukraine’s forces on "extremely sophisticated" Typhoons and F-35s would take too long but it did not oppose allies sending their own jets.
After completing a review, Prime Minister Rishi Sunak said a "prolonged stalemate" in the war in Ukraine "would only benefit Russia" and its President Vladimir Putin, a government spokesman said.
“The UK's Typhoon and F-35 fighter jets are extremely sophisticated and take months to learn how to fly. Given that, we believe it is not practical to send those jets into Ukraine," Mr Sunak's official spokesman said.
He said the "length of time" in training was the limiting factor, rather than opposition to supplying Ukraine with the lethal weaponry.
Mr Sunak said they must seize an "opportunity to accelerate UK support" and give Ukraine "the best chance of success and make the most of the window of opportunity where Russian forces were on the back foot".
Poland is not in talks to send F-16 fighter jets to Ukraine, a deputy defence minister said on Tuesday. "There are no official discussions on transferring F-16s at the moment," Wojciech Skurkiewicz said.
There has been no indication that a decision on sending warplanes to Ukraine might come any time soon and no sign that western countries have changed their previous stance on the issue.
Some leaders have expressed concern that the move could escalate the nearly year-long conflict and draw them deeper into the war.
German Chancellor Olaf Scholz appeared to balk at the prospect of providing fighter jets and suggested the entire discussion might come down to “domestic political motives” in some countries.
Dutch Prime Minister Mark Rutte on Monday said there were “no taboos” in efforts to help Ukraine. But he added that sending jets “would be a very big next step”.
France will ship 12 more Caesar lorry-mounted field guns and new air defence equipment to Ukraine to bolster the fight against Russian invaders, Defence Minister Sebastien Lecornu said Tuesday.
The artillery pieces, adding to 18 already delivered, would be financed from a €200 million ($217m) pot France set up to fund arms for Kyiv, Mr Lecornu said alongside Ukrainian Defence Minister Oleksiy Reznikov.
Along with other western mobile guns, such as the German Panzerhaubitze 2000, the Caesar was credited last year with helping Ukraine to strike targets deep behind Russian lines, undermining Moscow's offensive.
The truck-mounted 155mm guns can be set up, fire a highly accurate volley at ranges of up to 40km, and shift position before the enemy can find them and fire back.
Mr Lecornu said the new batch of howitzers would be delivered "in the coming weeks" and had originally been ordered by Copenhagen.
Denmark has also pledged its entire 19-strong group of the French-made guns to the Ukrainian war effort.
"We've now reached a mass [of Caesar cannons] that cannot be overlooked," Mr Lecornu said.
Paris will also give Kyiv a Ground Master 200 radar capable of detecting enemy aircraft at distances as far as 250km, including low-flying drones.
The radar can also detect incoming rockets or artillery fire and warn friendly troops.
Ukraine's Prime Minister, Denys Shmyhal, said on Tuesday that a summit with the EU would take place in Kyiv on Friday, sending a "powerful signal" to Moscow and the world.
"The Ukraine-EU summit will be held in Kyiv on February 3," Mr Shmyhal told a government meeting, calling the event "extremely important" for Kyiv's bid to join the European bloc.
"The fact that this summit will be held in Kyiv is a powerful signal to both partners and enemies."
No details were provided on who would be attending from the EU.
Mr Shmyhal said that another key event will take place on Thursday, when consultations between the government of Ukraine and the European Commission will take place "for the first time in our history".
Ukraine gained EU candidacy status in June last year.