Two weeks before negotiators hope to reach a nuclear agreement with Iran, the U.S. ambassador to the United Nations tried to reassure Congress that sanctions could be easily re-imposed on Tehran if it cheats.

Rep. Ed Royce, chairman of the House Foreign Affairs Committee, said lawmakers fear that Russia and China, which have veto authority as permanent members of the U.N. Security Council, would make it difficult to snap sanctions back into place for non-compliance.

Samantha Power told the House Foreign Affairs Committee that she couldn't go into detail about what kind of snap-back mechanism would be in place at the United Nations. But she said the U.S. and its partners will "not allow snap-back to be left in the hands of Russia or China."

Negotiators from the six-nation group — United States, Russia, Britain, France, China and Germany — and Iran are working to clinch a final accord by the end of the month to curb Iran's nuclear activity for a decade in exchange for lifting sanctions. The six powers want long-term caps on Iranian nuclear programs that could be used to make weapons. Tehran denies any interest in atomic arms and is seeking an end to sanctions in exchange for nuclear concessions.

The talks are continuing to address the pace at which the United States and other countries would provide Iran relief from international sanctions — Tehran's biggest demand — and how to return punitive measures into place if the Iranians are caught in noncompliance.

President Barack Obama has cited the snap-back mechanism as a main defense of the proposed pact from sharp criticism from Congress and some American allies.

Power also addressed committee members' concerns about what Royce said was the "U.N.'s continuing anti-Israel bias," especially in the organization's human rights council.

"More disturbing is that the Obama administration seems on the brink of discarding decades of bipartisan support of Israel against the U.N. onslaught," said Royce, R-California.

After Israeli Prime Minister Benjamin Netanyahu said he would not allow a Palestinian state to be established on his watch, Obama said he would reassess U.S. policy toward Israel.

Netanyahu has since tried to walk back his statement, saying he was committed to a Palestinian state under certain conditions. But U.S. officials say his statement makes it harder to fight back against criticism made against Israel.

Power defended the administration, saying that "day in and day out, we push back against efforts to de-legitimize Israel at the U.N., and fight for its right to be treated like any other nation."

Asked specifically if the U.S. would veto any U.N. resolution that forces or imposes a two-state solution on Israel, Power said it was "perilous to make blanket statements" about hypothetical scenarios, but that the U.S. would oppose any resolution that undermined Israeli security.