Abbas says he has a promise from Washington that if he agrees to direct negotiations, Israel would prolong a partial moratorium on West Bank settlement building that is due to expire in September.
But he wants to know in advance what shape and size of a future Palestinian state Israel is prepared to discuss in direct negotiations, and whether it is ready to quit the Jordan Valley and entrust security there to a third party.
Abbas also seeks clearer assurances from the United States.
"We are not against direct negotiations," he told reporters in Ramallah after publication of a closed-door speech to his Fatah movement outlining slow progress so far in indirect negotiations mediated by US envoy George Mitchell.
"If there is progress by July 28, we will present it to the Arab League. If there is no progress, we will tell the League that we will continue with the proximity talks until the end of the four-month mandate we received," Abbas said.
The Arab League committee that approved the indirect talks is due to convene in Cairo on July 29.
In a message relayed by Mitchell, President Barack Obama said that if the Palestinians went to direct negotiations, the so-called moratorium would be extended and "no single house would be built on Palestinian land during the extension," Abbas told Fatah's Revolutionary Council.
Israeli Prime Minister Benjamin Netanyahu has voiced reluctance to extend the 10-month freeze, a move that could put strains on his governing coalition, which is dominated by pro-settler parties, including his own.
But Netanyahu has not spelled out what he intends to do, raising speculation in Israel of a possible de facto moratorium if direct peace talks begin.
"FEW AND INSUFFICIENT"
Abbas said Obama used language reaffirming that "we believe that the occupied territories that will be discussed are the Gaza Strip, the West Bank, Jerusalem, the Dead Sea, and the Jordan Valley."
But Abbas said "the language used was less clear than the language that we had" during the previous Bush administration and the ideas put foward were "few and insufficient, and require a lot of clarification."
"We need to check the issue of halting settlement in a clear and well-defined way. There should also be clear terms of reference for the negotiations and at least we should define the 1967 borders. If this happens, we could go to direct negotiations," Abbas told Fatah members.
The Palestinian leader's credibility has been eroded by the failure of past negotiations and he is under pressure from Fatah to avoid more direct talks with Israel that could be fruitless.
Abbas suspects Netanyahu's government is not ready to make peace on terms the Palestinians can accept. But he faces pressure from Obama to embark on direct talks, which Netanyahu says he is willing to begin right away.
Abbas's Palestinian Authority depends on the political and financial backing of Western states who are impatient for real progress toward a treaty ending the 62-year-old Middle East conflict and establishing a Palestinian state.
Israel's government says indirect talks are wasting time and has criticized Abbas for setting preconditions for direct talks.
