(CNN)Ukrainian forces have so far managed to stave off Russian ground forces massed less than 20 miles north of the embattled capital Kyiv -- but though they've defied US intelligence predictions that the city would likely fall within one to four days of a full-scale Russian assault, US officials warn that Russian President Vladimir Putin could imminently increase the intensity of the attack. Stiffer than expected Ukrainian resistance and Russia's own logistical missteps have bogged down Russia's advance, US and western officials say, sparking a fragile optimism. The US has also continued sharing downgraded intelligence with the Ukrainians, including about...