A key U.N. climate fund hit a snag Thursday as major donors including the United States and Japan failed to meet a deadline for converting their pledges of climate aid into signed agreements.

The Green Climate Fund — intended to become the main international channel of climate finance for developing countries — said it had inked deals for 42 percent of the $10 billion that governments pledged last year. It needs to hit 50 percent to start considering projects for funding — something it hopes to do before a major climate summit in Paris in December.

Executive Director Hela Cheikhrouhou urged governments that haven't finalized agreements to do so "urgently" so the fund can start allocating money.

The U.S. pledged $3 billion to the fund while Japan promised $1.5 billion.