By David Morgan and Ayesha Rascoe

WASHINGTON (Reuters) - President Barack Obama on Thursday said it was time for Congress to lay aside politics and to act to provide additional money to combat the Zika virus before government funding dries up.

"Our experts at the CDC, the folks on the front lines have been doing their best in making due by moving funds from other areas, but now the money we need to fight Zika is rapidly running out," Obama said at a press conference at the Pentagon.

He warned that development of a vaccine for the virus could be delayed if Congress does not provide any more money and urged Americans to contact lawmakers to pressure them to take up the issue.

Concern over the threat from Zika, which can cause a birth defect called microcephaly marked by small head size that can lead to severe developmental problems in babies, has risen since Florida authorities last week detected the first signs of local transmission in the continental United States.

Zika funding remains stalled six months after Obama asked the Republican-led Congress to approve $1.9 billion in emergency funds.

Forty-one Democratic U.S. senators on Thursday sent a letter to Senate Majority Leader Mitch McConnell and House of Representatives Speaker Paul Ryan urging them to summon lawmakers back from their summer recess to vote on emergency funding to fight the virus.

But, Ryan accused Democrats of obstructing the funding and said the Obama administration has failed to spend existing funds to prevent the spread of the mosquito-borne virus.

Congress has interrupted its own recesses at least eight times since 1998, most recently in 2013 when lawmakers returned early to debate the use of military force in Syria, according to congressional records.

The latest round of finger-pointing indicated there was little chance lawmakers would cut short their seven-week summer break to vote on Zika funding.

A $1.1 billion compromise failed after House Republicans attached language that would place restrictions on abortion and defund part of Obama's signature 2010 healthcare law.

Senate Democrats, who blocked the measure twice before Congress left Washington last month, want Republicans to agree on a new funding measure that drops those provisions, a step Republicans reject.

"We need the White House and Senate Democrats to drop politics and put the public's health first. We hope for a change of heart, and soon," Ryan wrote in an opinion piece in the USA Today newspaper.

McConnell spokesman Don Stewart invited Democrats to allow the current bill to pass by unanimous consent at a perfunctory session on Friday.

(Additional reporting by Richard Cowan; Editing by Will Dunham and Bernard Orr)