U.S. debt ceiling could bind in mid-February - study

* U.S. would default if $16.4 trillion limit not raised

* Research group predicts Feb. 15-March 1 deadline

* Assumes Washington will not postpone tax filing season

WASHINGTON, Jan 7 (Reuters) - The United States may run out of tools to avoid a default as early as February 15, piling more pressure on lawmakers and the Obama administration to resolve their fiscal battles, a study found on Monday.

The U.S. Treasury in late December began shuffling funds in order to keep paying the government's bills, buying time for lawmakers to raise the $16.4 trillion legal limit on the nation's debt. It confirmed the ceiling was scraped on Dec. 31.

Treasury Secretary Timothy Geithner predicted the maneuvers -- which include pausing some investments in federal pension and health benefit funds -- would normally buy the government about two months before the debt limit became binding.

Possible delays in the tax filing season could extend that time further, as the government would get more time to refund taxes, Geithner said at the time.

But the Bipartisan Policy Center, a Washington-based think-tank, said the Treasury may run out of funds between Feb. 15 and March 1, based on an analysis of the government's daily and monthly cash flows.

"It will be difficult for Treasury to get beyond the March 1 date in our judgment," Steve Bell, senior director of the Center's Economic Policy Project, said in a statement.

The Center said it assumes taxes will be filed as scheduled.

In years past, Congress has periodically raised the debt ceiling without much fuss to take into account the budget deficits from its decisions on taxes and spending.

But the limit has become a hostage in the high-stakes fiscal battle roiling the U.S. capital, with Republicans refusing to raise it unless Democrats agree to deep spending cuts to tame the ballooning national debt.

The Bipartisan Policy Center said the Treasury has fewer tools now to stave off a default than it did in the summer of 2011, when battles over raising the debt ceiling pushed the United States to the brink of default and hit its credit rating.

At the time, the Treasury's tools lasted roughly three months, from May until early August.

If the government runs out of cash, it may have to choose between making interest payments on its debt, refunding taxes, or making payments for Medicare and Medicaid -- all due in late February and early March.

(Reporting by Anna Yukhananov; Editing by Dan Grebler)