UPDATE 5-Trump supports massive U.S. funding bill, shutdown looms

(Adds White House statement)

WASHINGTON, March 21 (Reuters) - President Donald Trump in a discussion with congressional leaders on Wednesday backed a $1.3 trillion U.S. spending bill, which aides said will include new funds for border security, the Pentagon, infrastructure and fighting Russian election hacking.

The White House said in a statement that Trump, Senate Republican leader Mitch McConnell and House of Representatives Speaker Paul Ryan "discussed their support for the bill," intended to avert a government shutdown on Friday.

The president had a discussion with Speaker Ryan and Leader McConnell, where they talked about their shared priorities secured in the omnibus spending bill," the White House said.

While it has not been formally unveiled, congressional aides said the measure would also contain gun legislation and a fix to a "grain glitch" in last year's sweeping tax law overhaul.

Other components will be $10 billion in infrastructure spending for highways, airports and railroads and an increase of $2.8 billion to fund opioid addiction treatment, prevention and research, a congressional source said.

Republican and Democratic congressional aides told Reuters that leaders plan to unveil their spending plan on Wednesday.

A senior Republican source said legislation to improve existing background checks on gun buyers would be included, as would a measure to further help schools prevent gun violence.

These steps, following the Feb. 14 mass shooting at a Parkland, Florida, high school and Tuesday's shooting at a school in Maryland, fell short of Democrats' calls for far tougher background checks and a ban on assault-style weapons.

The bill was expected to fund the government through Sept. 30, taking budget squabbling off the table in the Republican-dominated Congress for the next several months and allowing lawmakers to focus on their November re-election efforts.

Lawmakers have been arguing since early 2017 about funding for the current fiscal year, which began last Oct. 1. Since then, several temporary funding measures have been enacted, but only after two brief government shutdowns occurred due to Congress' inability to pass appropriations bills in a timely way.

Ryan's communications office issued a statement saying the legislation had been finalized but full details were not yet made public.

"We do have a few language points that need to clear up but we have made good progress," House Democratic leader Nancy Pelosi said in a statement.

Without action by Congress, temporary funding for some agencies and programs would expire at midnight on Friday. (Additional reporting by Susan Heavey, Lisa Lambert Editing by Kevin Drawbaugh, Bill Trott and Leslie Adler)