Ukrainian President Volodymyr Zelensky pushed back against concerns from US and NATO officials that his military would need significant training before receiving advanced weapons from other countries, telling CNN his forces need weapons quickly -- and can learn to use them just as fast.

"I've heard these tall tales that we would need months to train our troops to use new tanks. OK, give us a Soviet-era tank," Zelensky said in an exclusive interview with CNN's Jake Tapper on Friday. "We are prepared to use any type of equipment, but it needs to be delivered very quickly. And we have the ability to learn how to use new equipment. But it needs to come fast."

Zelensky said in the interview from the office of the president in Kyiv that the weapons the US and other countries have provided to date, including the Biden administration's new $800 million security assistance package, have helped his country resist Russia's invasion. But he said that more would be needed, and his country's trainers would be able to get his forces up to speed quickly.

"I have heard many times from certain states that did not want to give us weapons quickly because our soldiers are not ready, from a technical standpoint, to use them," he added. "But instructors of such equipment, our instructors, will get our troops ready to fight in them. If it's a plane, for example, pilots can be ready in two weeks. Whether it's Kamikaze drones, artillery, Howitzers, or (multiple launch rocket system) MLRS complexes, we have very smart people for these. We've had training with NATO countries."

Zelensky's interview with CNN comes as Russia is preparing for a new phase in the war after failing to capture Kyiv. Russian forces are preparing to ramp up their offensive in the eastern and southern parts of Ukraine, including Mariupol. Ukrainian forces rejected Russia's ultimatum to surrender in Mariupol on Sunday.

In the interview, Zelensky said Ukraine would give not give up territory in the east to Russia and is prepared to fight, while also warning that the world should be prepared for the possibility that Russian President Vladimir Putin could use tactical nuclear weapons.

Zelensky vows forces in Mariupol will not surrender

Zelensky told CNN that Ukraine's forces remaining in the besieged city of Mariupol will not surrender to Russia, saying that he does not believe Russia will allow people to leave Mariupol unharmed.

"They say they are ready to let all the military go if they surrender. But they are not going to surrender. They don't want to do so and this is why it is a complicated and tragic situation because the military don't want to surrender. And without it, Russians are not ready to let them go," Zelensky said. "When you can make an agreement with Russians to let them go, unarmed, but after, what they will do is shoot them dead. That is why no one trusts Russia now."

Zelensky added that Ukraine wanted to take away the wounded from Mariupol but Russian forces would not let them.

"We wanted to take away the wounded. We talked about it being a humanitarian mission. Give us the wounded back," he said. "We even made plans for Turkey to be a mediator and get the wounded, civilians and the military. They don't let them out because we understand Russia just wants to shoot them dead."

"We wanted to take away the wounded. We talked about it being a humanitarian mission. Give us the wounded back!" he said. "We even made plans for Turkey to be a mediator and get the wounded, civilians and the military. They don't let them out because we understand Russia just wants to shoot them dead."

The situation in Mariupol is bleak. Much of the city has been destroyed by Russian shelling. Civilian structures targeted included a maternity hospital and a theater where up to 1,300 people were seeking refuge.

Though many have fled, an estimated 100,000 people still remain in the city and its immediate surroundings, which are reported to be largely under Russian control.

Zelensky has pushed for European countries to cut off Russian oil. Asked about the Russian fuel that flows through pipelines in Ukraine, Zelensky said, "Yes, we understand this problem."

"We understand how much money Russia makes on energy carriers. And yes, first of all, we are calling for an oil embargo because they make 1 billion a day on oil. And we are calling for a gas embargo afterwards," Zelensky said. "And we are fighting together and looking into these options. But this is transit. We have a transit agreement. That is why it is necessary to have an agreement between Europe and Russia, so they have an embargo on gas supply. And then there will be no transit."

Zelensky added that Russian forces have targeted Ukrainian gas pipelines to try to suggest to European countries they can't rely on Ukraine.

"This is what Russia is aiming at. They are trying to act this way in order to show that Ukraine is not able to safely supply gas," Zelensky said. "They are doing it and they have been doing it for a while now because they want an opportunity to reopen Nord Stream 2. And we are constantly observing it. I've already talked about it to our European partners, I don't want to say what leaders I talked to. I told them, please put embargo on gas, please. We are ready to lose money. Money is not the most important thing for us. People are most important."

- By Jeremy Herb, CNN