President Donald Trump may have expected a Russian shudder of fear to greet his threat of “very severe tariffs” on the country if it didn’t agree a ceasefire in Ukraine.

I STANBUL—President Trump’s ultimatum for Russian President Vladimir Putin is showing no signs of bringing Russia and Ukraine any closer to peace. The third round of direct talks between Ukrainian and Russian officials, held Wednesday in Istanbul, yielded few results. The two sides met for less than an hour.