KIEV, Ukraine—Momentum gathered in the West to censure Russia and shore up Ukraine's moribund economy, as the International Monetary Fund readied as much as $18 billion in rescue loans to help avert a financial collapse.

The IMF agreement—which calls for what Ukrainian officials described as painful budget cuts and other measures that will strain the country's fragile economy—will unlock additional aid from other donors. IMF official Nikolay Gueorguiev told a news conference at Ukraine's central bank in Kiev on Thursday. In...