Watch defence secretary Ben Wallace on war in Ukraine
Mr Zelensky also called for allies to provide “effective counteraction” against Moscow’s advances after holding talks with Boris Johnson on Friday morning.
Downing Street said the Prime Minister pledged further support “in the coming days” after the president updated him on the “terrible developments” in the Ukrainian capital
Defence Secretary Ben Wallace said more than 450 Russian troops had been killed, as a fierce resistance meant Moscow “failed” on the main objective on the first day of fighting.
Mr Johnson and Mr Zelensky spoke after Kyiv was hit by “horrific rocket strikes” and fighting reached the outskirts of the capital overnight.
After the call, the president said Ukraine “needs the support of partners more than ever”.
“We demand effective counteraction to the Russian Federation. Sanctions must be further strengthened,” Mr Zelensky added.
Mr Johnson unveiled the “largest and most severe” package of sanctions Russia has faced to punish Vladimir Putin, hitting further oligarchs and freezing Russian bank assets.
The Kremlin retaliated to the ban on Aeroflot planes touching down in Britain by banning UK flights to and over Russia on Friday.
Mr Johnson assured Mr Zelensky that “the world is united in its horror at what Putin is doing” and praised the “bravery and heroism of the Ukrainian people”, No 10 said.
“The Prime Minister committed to provide further UK support to Ukraine in the coming days as the people of Ukraine and the world continue to demonstrate that Putin cannot act with impunity,” a statement added.
The Kremlin’s long-feared assault began in the early hours of Thursday, but the British Ministry of Defence said a “fierce resistance” was holding up the Russian advance.
Mr Wallace told Sky News: “Our assessment as of this morning is that Russia has not taken any of its major objectives, in fact it is behind its hoped-for timetable.
“They’ve lost over 450 personnel. One of the significant airports they were trying to capture with their elite Spetsnaz has failed to be taken. In fact, the Ukrainians have taken it back.
“So, I think contrary to great Russian claims and indeed President Putin’s sort of vision that somehow the Ukrainians would be liberated and would be flocking to his cause – he’s got that completely wrong. The Russian army has failed to deliver on day one its main objective.”
Mr Wallace ruled out Britain helping enforce a no-fly zone over Ukraine because the RAF fighting Russian jets would trigger a “war across Europe”.
“He is trying to invade Ukraine. He won’t stop after Ukraine. He will use everything in the Baltic states. He doesn’t believe the Baltic states are really countries,” he told BBC Radio 4’s Today programme.
“And we will have to stand up to it. Now, I cannot trigger a European war and I won’t trigger a European war but what I will do is help Ukraine fight every street with every piece of equipment we can get to them, and we will support them, and that is the reality.”
Britain, the US and the EU have hit Moscow with a concerted package of economic sanctions, but are facing calls to go further to exclude Russia from the Swift international payment system, a move backed by Mr Johnson but facing resistance in Europe.
Among the new UK sanctions introduced were measures to hit five further oligarchs, including the Russian president’s former son-in-law, and to target more than 100 businesses and individuals.
In the US, President Joe Biden was targeting Russian banks, oligarchs and high-tech sectors, while deploying more troops to Germany to bolster Nato.
Mr Zelensky said in an address early on Friday sanctions alone were not doing enough to deter Russia.ome Ukrainians who need sanctuary.”