The commander of US troops in Europe says Nato cannot rapidly deploy large forces to Eastern Europe in the way that Russia can.
Lt-Gen Ben Hodges was speaking to the BBC's Hardtalk programme during large Nato exercises in north-western Poland.
"The Russians are able to move huge formations and lots of equipment a long distance very fast," he said.
Nato needs to have that speed too, he said. "Three days' notification, we ought to be able to do that," he said.
More than 31,000 troops from 24 nations took part in Nato's Anaconda-16 exercises in Poland, from 7 to 17 June.
The day after they ended, German Foreign Minister Frank-Walter Steinmeier warned Nato against "sabre-rattling and warmongering", calling for exercises to be replaced with more dialogue and co-operation with Russia.
"Whoever believes that symbolic tank parades in Eastern Europe bring more security, is mistaken," he told Bild newspaper.