By Valerie Volcovici

WASHINGTON (Reuters) - The United States has paid $500 million into the United Nations' Green Climate Fund, the first tranche of the $3 billion it pledged as part of the commitments it made in the December Paris Climate Agreement, the State Department confirmed Monday.

"This grant is the first step toward meeting the president’s commitment of $3 billion to the GCF, and shows that the United States stands squarely behind our international climate commitments," a State Department spokesman said.

The United States in 2014 pledged $3 billion for the GCF, to be used by poor and climate-vulnerable countries for transformational projects to adopt cleaner energy technologies and build their resistance against the impact of climate change.

But the ability of the United States to deliver the funds had been in doubt because Republican members of Congress had threatened to block federal funds for climate aid as part of their efforts to undermine U.S. participation in the Paris agreement. They said Congress first needs to scrutinize details of the accord before it releases funds.

But lawmakers did not block the funds in December after they wrapped up a sprawling budget deal to keep the U.S. government operating through next September.

U.S. officials has said the United States is committed to meeting its international commitments on climate change, even after the Supreme Court last month froze the centerpiece of the country's climate change strategy, the Clean Power Plan.

The board of the GCF is meeting this week at its headquarters in Songdo, South Korea, for the first time since the Paris Agreement was concluded in December.

The fund currently has $10.3 billion in pledges and has a goal to spend $2.5 billion on projects in 2016.

Héla Cheikhrouhou, the fund's executive director, told Reuters earlier this month she will ask the board to approve an increase of between 80 and 120 new staff to meet the spending target.

"We need to have the tools and we need to have the people in place before we scale up further," she told Reuters.

(Reporting by Valerie Volcovici; Editing by Lisa Shumaker)