(Adds details on states, background)

By Corbett B. Daly

WASHINGTON, Nov 10 (Reuters) - About 20 percent of struggling U.S. homeowners eligible for loan workouts have entered into a trial modification, but it is unclear how many of them will be able to keep the lower payments permanently, the Treasury Department said on Tuesday.

It said there were 650,994 active trial modifications through October under President Barack Obama's plan to help the housing market. That was up from 487,081, or 16 percent, participating through September.

The Treasury did not release figures for trial modifications that have been made permanent.

Most loan modifications result in lower monthly payments, although some lead to reduced principal on mortgages. Trial modifications were initially for three months, but the Treasury added 60 days, effectively making them last five months.

Borrowers need to submit more detailed documentation to make a loan modification permanent.

Under pressure from Congress, the Treasury released state-by-state details of the loan modifications.

"As this report demonstrates, struggling homeowners in every state now benefit from reduced monthly mortgage payments and have an opportunity to stay in their homes," said Michael Barr, Treasury assistant secretary for financial institutions.