UPDATE 2-ING has 28.7 bln euros of Alt-A mortgage exposure

(Adds details about Impac, American Home Mortgage)

NEW YORK, Sept 24 (Reuters) - ING Groep NV (ING.AS) said its exposure to "Alternative-A" mortgages as of July 31 was $40.46 billion (28.7 billion euros).

The Dutch banking group said on Monday in a filing with U.S. regulators that "Alt-A" mortgage exposure accounted for about 2 percent of its assets and was mainly through asset-backed securities.

The news comes as trouble from the U.S. mortgage market ripples through the world economy. Subprime mortgages, which are of lower credit quality than "Alt-A," have experienced surging delinquencies.

Mitsubishi UFJ Financial Group, (8306.T) Japan's biggest banking group, said on Friday that subprime mortgage defaults will force it to record a decline in value of some of its investment securities.

Alt-A home loans, or mortgages made to people with good credit histories who are often unable to document income or assets, have experienced rising defaults, but not to the same extent as subprime home loans.

But with markets jittery, many Alt A lenders in this arena have had their credit cut off, limiting them from making new loans.

Impac Mortgage Holdings Inc, IMH.N a U.S. lender that had focused on Alt-A loans, last week said it was firing 144 workers and will stop making the loans in which it had previously specialized.

American Home Mortgage Investment Corp. AHM.N, which also focused on Alt-A home loans, filed for Chapter 11 bankruptcy protection in August.

For ING, to the extent there are any losses on the underlying loans, other investors will take the initial losses. ING will be either last or next to last to take losses.

ING said in August that it had 3.2 billion euros of exposure to subprime mortgages through asset-backed securities.

"Alt-A" mortgages are considered less risky than subprime mortgages, but have still experienced an increase in delinquencies in the United States.

The loans are made to people with good credit histories, but who are often unable to document income or assets. In some cases, the loans were used to buy larger homes than the purchaser could afford. (Reporting by Dan Wilchins)