Moscow has claimed its forces observed the ceasefire, but media reports say both sides exchanged artillery fire in eastern Ukraine after Putin's deadline.
The US will send €3.5 billion in military weapons and other aid to Ukraine and its neighbours on NATO's eastern flank. Germany are also following suit.
Both Russian and Ukrainian presidents marked the occasion, with Vladimir Putin calling for a 36-hour ceasefire and Volodymyr Zelensky hailing Ukrainians for their fighting spirit.
CNN's Ben Wedeman reports from the town of Siversk, Ukraine, where a mobile post office has become a lifeline for the elderly as it delivers their state pensions to buy supplies from the handful of shops still open.