Britain has pledged to continue to supply arms to Ukraine's embattled military as the fighting with Russian forces reached the outskirts of the capital, Kyiv.
Armed forces minister James Heappey said Russians troops had not made the progress they might have hoped, with the main armoured columns still some way from the city.
He warned that the defenders faced "days, weeks, months more" of heavy fighting as Russian President Vladimir Putin strives to topple the Ukrainian government and impose his writ on the country.
"This is going to be a long slog. It is going to be brutal. We are going to see some horrendous things on our TV screens," Mr Heappey told BBC Breakfast.
With the noose tightening around his capital, a defiant President Volodymyr Zelensky refused an American offer to evacuate, insisting: "The fight is here".
However, Mr Heappey disclosed that the Ministry of Defence was working on plans to support a resistance movement and a government in exile if Ukraine was finally overrun.
"That is a decision for the National Security Council to take but it is something that the Prime Minister has asked us in the Ministry of Defence to look at and plan for," he told Sky News. On Friday, Defence Secretary Ben Wallace convened a meeting with 25 other donor nations who all agreed to supply arms or humanitarian aid to Ukraine.
Britain has already sent 2,000 anti-tank missile launchers and Mr Heappy said they were looking to get more weaponry to the country.
"We know what the Ukrainians want. We are doing our best to get it to them," he said. While the situation was "very grave", he said it was clear that the Russian advance was not going to plan in the face of stiff Ukrainian resistance.
He said the Kremlin had expected to take a slew of Ukrainian cities on day one of the invasion, while encircling Kyiv ahead of a full-scale assault.
However, so far the fighting in the capital had been confined to "very isolated pockets of Russian special forces and paratroopers" with the main armoured columns "still some way off", he added.
"That is a testament to the incredible resistance the Ukrainian armoured forces have put up over the last 48 hours or so," he told Sky News.
"Clearly the Russian plan is to take Kyiv but the reality is that the Ukrainians are thwarting them thus far.
"I think that will be a great cause of concern for President Putin and rather points to the fact that there was a lot of hubris in the Russian plan and that he may be awfully advised."
In its latest intelligence assessment issued on Saturday, the Ministry of Defence said the Russian advance had "temporarily slowed" probably due to "acute logistical difficulties" as well as the strength of the Ukrainian resistance.
But with Moscow having massed an estimated 150,000 troops on the border ahead of the invasion, Mr Heappey said people needed to be "clear-eyed" about what lay ahead.
"President Putin, if you listen to his speeches, look at his press conferences over the last two or three days, there has been a fanaticism in the language that he used, a fervour in the tone of his voice. He has gone all in on this," he told the BBC.
"I am afraid that that means that what is in front of Ukraine is bloody, brutal. We will do everything we can to help them resist but people shouldn't necessarily think that a happy ending is just around the corner."
On Friday, the UK, the US and the European Union all announced plans to impose personal sanctions on Mr Putin and his foreign minister Sergei Lavrov.
Labour leader Sir Keir Starmer said the Government needed to go "further and faster" on sanctions and he called for the immediate expulsion of Russia's ambassador to the UK.
"Russia is invading a European country. It isn't right that he is here. He should be told to go straight away," he said during a visit to Birmingham.
Mr Heappey said Britain was still working to try to secure international agreement on the "ultimate economic sanction" of excluding Russia from the Swift system for international banking transfers.
Like the US, ministers have ruled out deploying British troops, warning that it would mean starting a war with Russia.
However, they face pressure from Tory backbenchers to do more to support the Ukrainians in the face of Russia's attempt to dismember a European state. Tobias Ellwood, the chairman of the Commons Defence Committee, said Britain should consider imposing a no-fly zone over the western half of the country.
"We're actually reacting to events rather than getting ahead of them. And the big question is - when do you stand up to Putin?" he told ITV News.
"We need to reconsider this no-fly zone, let's say west of the Dnieper River, because that would change the optics here."
"If we don't stand our ground now, where will this go? And don't forget there are other adversaries around the world, namely China, watching very carefully how the West reacts here."
The Netherlands, on the other hand, will send anti-tank weapons to Ukraine, the Defence Ministry said on Saturday.
The Dutch government will supply 50 Panzerfaust-3 anti-tank weapons and 400 rockets, the ministry said in a letter to parliament. The Netherlands is also jointly considering with Germany sending a Patriot air defence system to a NATO battle group in Slovakia, it said.
The Netherlands is also jointly considering with Germany sending a Patriot air defence system to a NATO battle group in Slovakia, it said.
Based on requests from Ukraine "the Netherlands will provide 200 Stinger air defence rockets", the letter said.
"Along with our allies, the Defence Ministry aims to deliver these goods as quickly as possible."
The missiles are in addition to other equipment already promised by the Netherlands earlier this month, including rifles, ammunition, radar systems and mine-detecting robots.
Earlier on Saturday, the Netherlands said it was moving Dutch embassy staff from the western Ukrainian city of Lviv to Jaroslaw, across the border in Poland, due to deteriorating security.
Germany's army will transfer 1,000 anti-tank weapons and 500 "Stinger" class surface-to-air missiles to Ukraine to help it in its battle against Russia's invasion, the government said Saturday.
The government took this decision on Saturday, and the weapons from the German army's stocks will be delivered to Ukraine as soon as possible, spokesman Steffen Hebestreit said in a statement.
German Chancellor Olaf Scholz said Russia's military assault on Ukraine has been "a turning point" for Europe, threatening the international order established after World War II.
"In this situation, it is our duty to do our best to help Ukraine defend itself against Vladimir Putin's invading army. Germany stands side by side with Ukraine," he said in a statement.
Earlier in the day, Berlin also approved the Netherlands' request to send 50 German-made anti-tank weapons and 400 rockets to Ukraine.
A spokesman confirmed that the government also approved Estonia's request to transfer nine D30 howitzers to the Ukrainian army.
The recent decisions marked a dramatic shift in the government's policy, and came after growing criticism from Ukraine and Germany's NATO allies.
Until Russia's military attack on Ukraine, Berlin ruled out supplying arms to Kyiv, and also turned down requests from allies to send their German-made weapons to the country.
On the third day of Russia's assault, street clashes between Russian and Ukrainian troops continued in the outskirts of Ukraine's capital Kyiv.
While Ukrainian President Volodymyr Zelenskyy vowed to defend his country, West announced sanctions on Moscow, including measures targeting Putin and Foreign Minister Sergey Lavrov.
