KYIV—U.S. Secretary of State Antony Blinken said on Wednesday that Russia could launch a new attack on Ukraine at very short notice but Washington would pursue diplomacy as long as it could. On a visit to Kyiv to show support for Ukraine, Blinken said Ukrainians should prepare for difficult days. He said Washington would keep providing defense assistance to Kyiv and renewed a promise of severe sanctions against Russia in the event of a new invasion. The Kremlin said tension around Ukraine was increasing and it was still waiting for a written U.S. response to its sweeping demands for security guarantees from the West. The pessimistic statements highlighted the gulf between Washington and Moscow as Blinken gears up for a meeting with Russian Foreign Minister Sergei Lavrov on Friday that a Russian foreign policy analyst called “probably the last stop before the train wreck.” Blinken promised “relentless diplomatic efforts to prevent …