* Netanyahu says Iran is using negotiations "to buy time"
* Warning could foreshadow his talks with Obama this month
* Biden says window for diplomatic solution is closing
Israeli Prime Minister Benjamin Netanyahu warned on Monday that Iran was getting closer to being able to build a nuclear bomb despite sanctions and diplomacy and said a "clear and credible military threat" was needed to halt Tehran's program.
Speaking via satellite link from Jerusalem, Netanyahu used an address to an influential U.S. pro-Israel lobbying group to underscore Israeli impatience with Washington's strategy on Iran, a message that could foreshadow his talks with President Barack Obama on a Middle East visit later this month.
"Words alone will not stop Iran. Sanctions alone will not stop Iran. Sanctions must be coupled with a clear and credible military threat if diplomacy and sanctions fail," Netanyahu said to loud cheers at the American Israel Public Affairs Committee (AIPAC) policy conference in Washington.
Despite Netanyahu's tough rhetoric, the hawkish prime minister gave no indication that Israel was ready to act precipitously at a time when world powers have re-engaged with Iran in new negotiations and he himself is caught up in the delicate task of forging a new government after January's elections.
But Netanyahu's remarks showed that the latest round of international talks with Iran in Kazakhstan last week had done little to soothe Israeli concerns about Tehran's nuclear drive.
Netanyahu repeated his assertion that Iran was using the negotiations "to buy time" to press ahead with its nuclear program.
Netanyahu has strongly hinted at Israeli plans to strike Iran's nuclear sites if it deems peaceful options to have failed. He has pressed the Obama administration to set strict limits on Tehran's nuclear development that would trigger U.S. military action - a demand that has fueled tensions between the two close allies. Obama has resisted the demand.
Speaking before Netanyahu, U.S. Vice President Joe Biden insisted that Obama was "not bluffing" about Washington's determination to stop Iran from getting a nuclear weapon and said "all options including military force are on the table."
He repeated the administration's position that there is still time for a diplomatic solution, though he said "that window is closing."
IRAN 'CLOSER TO THAT RED LINE'
Netanyahu said Iran had not yet crossed a "red line" he set at the United Nations in September, when he said Tehran should not be allowed to amass enough medium-enriched uranium that, if purified further, would be enough to power a single warhead. He gave a rough deadline at the time of spring or summer 2013.
But he told AIPAC: "Iran is getting closer to that red line and its putting itself in a position to cross that line very quickly once it decides to do so ... We cannot allow Iran to cross that red line." However, Netanyahu stopped short of any explicit threat of Israeli military action.
Netanyahu's calculus on Iran is complicated by Israel's unsettled domestic politics. He is still struggling to forge a new coalition government after a surprisingly strong showing by centrist parties in January's elections.
At the talks in Almaty, Kazakhstan, the United States and five other powers offered Iran modest sanctions relief in return for Tehran curbing its most sensitive nuclear work. There was no breakthrough but the sides agreed to resume political discussions in early April.
Netanyahu has insisted that Iran, whose leaders have frequently threatened Israel, is using the negotiations to stall for time to continue development of nuclear bomb capability. Tehran denies it has any such aim. Israel is assumed to be the Middle East's only nuclear-armed power.
"The latest efforts at conciliation and some kind of agreement with the Iranians have failed," Republican U.S. Senator John McCain told the audience earlier. "It's very clear that they are on the path to having a nuclear weapon. I don't think it's whether, it's obviously a question of when."
Obama has repeatedly pledged to prevent Iran from acquiring a nuclear weapon but his refusal to take an even stronger stance with Tehran has contributed to a testy relationship with Netanyahu.
Even so, the situation has calmed considerably since Obama addressed AIPAC last year and issued a pointed warning against "loose talk" of war with Iran.
A senior Israeli official said that while the Netanyahu government had hoped for a tougher line at the negotiations by the so-called P5+1 - made up of the United States, China, France, Russia, Britain and Germany - it was resigned to awaiting the results of the next round of talks.
Iran may have lessened Israel's immediate sense of urgency by turning some of its 20 percent-pure uranium - which is considered to be only a short technical step away from weapons-grade uranium - into fuel rods for a research reactor.