"Clearly if the president decides to commit additional forces to Afghanistan, there would be an expectation that our allies would also commit additional forces," Geoff Morrell, a Pentagon press secretary, told a news conference.
Over the past three years, allies in the North Atlantic Treaty Organisation have consistently sent additional forces to Afghanistan following the deployment of US reinforcements, he said.
Mr Morrell said: "The bottom line is there is no denying that Nato has ponied up significant numbers of additional forces as we have added forces over the last several years."
About 68,000 American troops are stationed in Afghanistan along with about 42,000 other foreign forces in the Nato-led mission.
Mr Obama on Tuesday promised to "finish the job" in Afghanistan and is expected to announce next week that he will send more than 30,000 troops to Afghanistan.