Israeli Prime Minister Benjamin Netanyahu must present a concrete plan for a two-state solution of the Palestinian conflict to the U.S. Congress this month or face the prospect of fresh violence, a U.S. lobbyist said.

Jeremy Ben-Ami, director of J Street, touted as a left-wing version of the powerful pro-Israel AIPAC lobby group, said he was "trying to build a momentum" to stall a Palestinian plan to seek United Nations backing for statehood in September.

He told Reuters before the end of a visit on Sunday that he thought the only way to effectively delay the plan and reduce tensions stoked by surrounding Arab uprisings was for Netanyahu to chart a clear path to a two-state deal.

Ben-Ami said he feared Netanyahu's labelling of a reconciliation pact between Palestinian President Mahmoud Abbas's Fatah group and rival Hamas Islamists as a blow to peace may only further damage already-frozen diplomacy.

"I think now in the wake of the reconciliation there's a possibility that his whole case to the Congress might be ... not putting anything on the table," Ben-Ami said.

"In our meetings here we are urging, from our perspective, that the prime minister's initiative should be a serious plan," he said, adding that he thought Israel should wait to see the policies of a new Palestinian government before condemning it.

Ben-Ami said Netanyahu ought to present a deal along lines agreed in past years of negotiations, including proposed land swaps in exchange for settlement blocs Israel would keep.

"Put a proposal on the table that meets a bar of credibility, not a provisional state on 30 or 40 percent of the land, but a real state, and let them decide if they're serious about peace or not," Ben-Ami said.

THREAT OF VIOLENCE

If Palestinians proceed with plans to seek U.N. General Assembly endorsement for statehood in September without agreeing the new state's borders with Israel, violence may ensue once it becomes clear that the conflict hasn't really ended, he said.

"Frustration will be higher," Ben-Ami said. Such sentiment "leads to explosions and all you need is one match on the tinder and we're very worried about what that leads to."

He said Israel may itself face a global diplomatic "tsunami" including a stepped up drive for sanctions over its continued occupation of land captured in a 1967 war.

U.S.-brokered peace talks collapsed last year over the issue of Jewish settlements built on land Palestinians want for a state.

Netanyahu has not said what peace ideas he may broach in Washington, where he also meets President Barack Obama on May 20, ahead of his speech to Congress and uncertainty as to what he might say has grown since his condemnation of the Palestinian unity deal signed in Cairo last week.

Israel objects to any contacts with Hamas, a group that refuses to recognise the Jewish state or past peace accords.

Spokesmen for Netanyahu declined to comment on Ben-Ami's remarks. But an Israeli official speaking on condition of anonymity said:

"I am sure that everyone who listens to the prime minister's speech in Congress will be impressed by the prime minister's desire to move forward in peace with our neighbours."