Iran will be allowed to access an additional $2.8 billion of its frozen assets during a four-month period of extended talks with six powers on its nuclear program, but most sanctions against Tehran will remain in place, the United States said.

"Iran will not get any more money during these four months than it did during the last six months, and the vast majority of its frozen oil revenues will remain inaccessible," U.S. Secretary of State John Kerry in a statement released in Vienna on Saturday. "We will continue to vigorously enforce the sanctions that remain in place."

Earlier, Iran and the six powers agreed to extend nuclear talks by four months after they failed to reach a July 20 deadline for a long-term deal that would gradually lift the sanctions against Tehran that have crippled its economy in exchange for curbs on the Iranian atomic program.