U.S. envoy to the Middle East George Mitchell is frustrated by the conduct of Prime Minister Benjamin Netanyahu in the proximity talks with the Palestinians. Mitchell, who is due in Israel on Thursday for another round of talks in Jerusalem and Ramallah, has expressed to Netanyahu his wish to see more progress by Israel on core issues.

Netanyahu is due to meet with U.S. President Barack Obama at the White House in a week.

A senior U.S. administration official told Haaretz Monday that Mitchell is interested in seeing more "seriousness" in talks on the core issues. "We want things to move faster and that there will be more progress on a number of issues," the senior U.S. official said. "To date there has been insufficient progress."

The senior U.S. official also said that the administration would like Netanyahu to show more willingness for substantive discussions on core issues, and to see the Palestinians moving toward direct talks with Israel.

There have been four rounds of proximity talks so far, during which Mitchell shuttled between Ramallah and Jerusalem. During talks with Palestinian Authority President Mahmoud Abbas and Netanyahu, the two informed the U.S. envoy of their positions on the various core issues. However, the Palestinian side has presented far more detailed positions.

"We did not offer a list of questions to any side," the senior U.S. official said. "Each side presented different things and on this basis we asked questions."

The senior U.S. official said that the issues that were raised during talks between Netanyahu and Abbas and Mitchell were not identical - however, there was some overlap. "No one is refusing to discuss things that Mitchell wants to talk about," the American source said.

A senior Israeli source updated on some of the content of the proximity talks said that the American frustration stems from the fact that Netanyahu has so far not given any clear answers on the borders of the future Palestinian state. During the past three rounds of proximity talks Netanyahu opted to dedicate much of the meetings to relatively peripheral issues, like water, the economic relations between Israel and the Palestinian state, and the development of a "culture of peace" in a future Palestinian state.

The PM's bureau said Monday that "Israel is conducting the proximity talks very thoroughly, on a variety of issues, in order to move as quickly as possible to direct talks."