President Donald Trump said Wednesday evening he's imposing "tremendous" sanctions on Russia's two biggest oil companies and said he hopes they will help make Russian President Vladimir Putin "reasonable" about ending the war.

"They'll certainly have an impact. They're massive sanctions and sanctions on oil," Trump said to reporters during an Oval Office meeting with NATO Secretary General Mark Rutte.

The sanctions against Rosneft and Lukoil, as well as dozens of subsidiaries, followed months of bipartisan pressure on Trump to hit Russia with harder sanctions on its oil industry.

"Hopefully it'll push, hopefully he'll become reasonable," Trump said of Putin, days after the two leaders announced that their talks were put on hold.

Meanwhile, as Ukrainian President Volodymyr Zelenskyy sought more foreign military help, Trump defended his decision to not provide Ukraine with Tomahawk missiles because it would take too long to train the Ukrainian military to use them.

The president said it would take "minimum of six months" or even a year to teach the long-range missile system to Ukrainians, adding that there is a "tremendous learning curve."

The Trump administration had previously considered the possibility of providing the weapons to Ukraine in order to help stop the war there started by Russia.

But Trump shut down the option after speaking with Russian leader Vladimir Putin by suggesting that the U.S. needed to preserve its supply of Tomahawk missiles.