(Adds White House and congressional comments on possible new sanctions)

By Lesley Wroughton

WASHINGTON, Nov 18 (Reuters) - U.S. Secretary of State John Kerry on Monday pressed Iran to finalize an agreement that can prove to the world its nuclear program is peaceful, but said he has "no specific expectations" for talks in Geneva this week between major powers and Iran.

The White House said President Barack Obama will meet with Senate leaders on Tuesday to press his case that lawmakers should not adopt any further economic sanctions on Iran over its nuclear program to allow the international talks a chance to succeed.

Last week, a senior U.S. official said the six major powers and Iran were getting closer to an initial agreement, but Kerry appeared to tamp down expectations two days before talks resume.

"I have no specific expectations with respect to the negotiation in Geneva except that we will negotiate in good faith and we will try to get a first-step agreement," Kerry told a news conference with Turkish Foreign Minister Ahmet Davutoglu.

Kerry said he hoped that "Iran will understand the importance of coming there prepared to create a document that can prove to the world this is a peaceful program."

"I am not going to negotiate this in public. We all need to be respectful of each others' processes here and positions - and so it's best to leave that negotiation to the negotiating table," he added, declining to discuss details of a proposal under discussion.

The six world powers are negotiating a proposal that would ease sanctions on Iran if it suspends some parts of a program that many countries, particularly in the West, fear is aimed at developing a nuclear weapons capability.

The talks between Iran and the so-called P5+1 powers, comprising the United States, Britain, China, France, Germany and Russia, resume in Geneva on Wednesday. They will try to reach a first-step agreement to end a 10-year deadlock over Iran's nuclear program.

Iran has denied that it wants to develop an atomic weapons capability and insists its nuclear ambitions are limited to the peaceful generation of electricity and other civilian uses.

State Department spokeswoman Jen Psaki said Kerry is open to participating in the upcoming talks "only if it makes sense" but will not decide until the talks get underway.

Talks between the P5+1 and Iran ended last week without an agreement, although the sides appeared to be close to a deal.

CONGRESSIONAL ACTION ON SANCTIONS

Obama urged Congress last week to hold off on new sanctions and sought to reassure lawmakers that any easing under the proposed deal would be modest and could be quickly reversed if Iran shows it is not serious about curbing its nuclear program.