UPDATE 1-Europe Won't Let Monetary Union Fail-US' Geithner

* But says Europe must move more aggressively

* Says US financial system stronger than 6-18 months ago (Adds comments from CNN interview)

WASHINGTON (Reuters) - U.S. Treasury Secretary Timothy Geithner said on Tuesday that Europe will not let its monetary union fail, but must move more aggressively to erect the financial capacity to safeguard it.

"They're not going to let the euro break up. They're not going to let European monetary union fall apart," Geithner said in an interview with "CNN's Erin Burnett OutFront" show.

"They're going to stand behind their financial system. They're going to take the steps to move closer to fiscal union, with the broader disciplines on budget policy you need to make monetary union work," Geithner added.

He said European policymakers are now talking about a more comprehensive, stronger financial firewall to support their financial system.

"But they've got to more quickly move, with more force," he added.

Asked whether the U.S. government should bail out a bank the size of Morgan Stanley if one were to be crippled by the European crisis, Geithner said, "Nobody in my position could answer a question like that."

He added, however, that the U.S. financial system was in a much stronger position than six months ago, 12 months ago and 18 months ago to weather pressures from Europe and slowing growth around the world. (Reporting by David Lawder; Editing by Gary Hill)