WASHINGTON - US President Barack Obama on Monday told Palestinian leader Mahmud Abbas that both he and Israeli leaders must make tough political decisions and take "risks" for peace. Meeting Obama at the White House, Abbas said Israel's release of a fourth tranche of Palestinian prisoners by March 29 would show how serious Prime Minister Benjamin Netanyahu was about extending peace talks."As I said to Prime Minister Netanyahu when he was here just a few weeks ago, I believe that now is the time to embrace this opportunity," Obama said as he sat side-by-side with Abbas in the Oval Office."It is very hard, very challenging. We are going to have to take some tough political decisions and risks if we able to move forward," Obama said. The US leader wants Abbas to agree to a US framework to extend peace talks past an end-of April deadline. Little tangible progress has been made in the past seven months.He said that everyone already understood the shape of an "elusive" peace agreement between Israel and the Palestinians, saying it would be based on 1967 lines with mutual land swaps.Abbas did not directly address the Israeli government's demand for the Palestinians to recognize Israel as a "Jewish" state. He noted through a translator that the Palestinians had recognized Israel's legitimacy in 1988 and in "1993 we recognized the state of Israel." Abbas also noted the agreement that the Palestinians have with Israel on the release of a fourth batch of prisoners by March 29."This will give a very solid impression about the seriousness of these efforts to achieve peace," Abbas said. "We don't have any time to waste. Time is not on our side, especially given the very difficult situation that the Middle East is experiencing and the entire region is facing," he said. Israeli ministers said last week that they would have difficulty approving the prisoner release if agreement was not reached to extend the peace talks. Israel committed to the release of 104 Palestinian prisoners in four tranches when talks were launched in July. It has so far released 78 of those in three batches. Ahead of the White House talks, thousands of Palestinians rallied in West Bank cities to show support for Abbas."We're here today to stand up to pressures upon us and make sure president Abbas adheres to his convictions," said Nasser Eddin al-Shaer, a former Palestinian education minister and member of Fatah's Islamist rivals Hamas, at a 5,000-strong rally in the northern West Bank city of Nablus.Obama told Netanyahu when they met at the White House on March 3 that the peace framework cannot be simply a deal agreed by Israel and the United States and then presented to the Palestinians as a take-it-or-leave-it offer.But officials also privately say that the Palestinians will be required to make concessions on issues like the return of refugees and borders if they are to secure a state at long last.However, despite intensive diplomacy by Secretary of State John Kerry, the two sides appear to have made little progress since the talks resumed in July after a three year freeze.Abbas met Kerry on Sunday for what a senior State Department official said were "frank and productive" discussions."We are at a pivotal time in the negotiations and while these issues have decades of history behind them, neither party should let tough political decisions at this stage stand in the way of a lasting peace," the official said.The most nettlesome issues in the peace process include the contours of a future Palestinian state, the fate of Jerusalem and Palestinian refugees, Israeli settlements, security and mutual recognition.The Palestinians want borders based on the lines that preceded the Six-Day War in 1967, when Israel captured the West Bank, including now-annexed Arab east Jerusalem.They have also insisted there should be no Israeli troops in their future state.But Israel wants to retain existing settlements it has built inside occupied Palestinian territory over the past decades. It also wants to maintain a military presence in the Jordan Valley, where the West Bank borders Jordan.