* Netanyahu says Iran is using negotiations "to buy time"
* Says al Qaeda, Hezbollah seeking Syria's chemical arms
* Netanyahu warning could foreshadow Obama talks this month
* Biden says window for Iran diplomatic solution is closing (Recasts with Biden speech, adds quotes, details throughout)
By Matt Spetalnick and Paul Eckert
WASHINGTON, March 4 (Reuters) - U.S. Vice President Joe Biden insisted on Monday that President Barack Obama was not bluffing about using force to thwart Iran's nuclear ambitions if all else fails, even as Israeli Prime Minister Benjamin Netanyahu called for a "credible military threat" against Tehran.
Seeking to reassure Israel and its U.S. supporters just weeks before Obama visits the Jewish state, Biden cautioned that all options, including sanctions and diplomacy, must be exhausted to ensure that the international community will be supportive if military action is deemed necessary.
But Netanyahu, speaking moments later via satellite from Jerusalem, used his address to America's largest pro-Israel lobby to underscore Israeli impatience with U.S. strategy on Iran, a message that could foreshadow his talks with Obama.
"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 the 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.
Netanyahu's remarks showed that the latest round of international talks with Iran in Kazakhstan last week had done little to soothe Israeli concerns. It is message he is likely to deliver face-to-face when he meets Obama, with whom he has had a notoriously testy relationship.
Despite that, Biden honed in on Obama's assertion in his 2012 AIPAC speech that he was ready to use force as a last resort to prevent Iran from getting a nuclear weapon. Tehran denies it is seeking one.
"President Barack Obama is not bluffing," Biden said to a standing ovation. "We are not looking for war. We are looking to and ready to negotiate peacefully, but all options, including military force, are on the table."
Netanyahu, who has hinted at Israeli plans to strike Iran's nuclear sites if it deems peaceful options to have failed, said Tehran was moving ever-closer to bomb capability and was using the negotiations to "buy time."
He has pressed the Obama administration to set strict limits on Tehran's nuclear development that would trigger a U.S. military response, a demand that has fueled tensions between the two close allies. Obama has resisted setting such an ultimatum.
Biden urged caution to avoid losing international solidarity against Iran, which faces possibly the toughest sanctions ever assembled. "If, God forbid, the need to act occurs, it is critically important for the whole world to know we did everything in our power, we did everything that reasonably could have been expected to avoid any confrontation," Biden said.
He said there was still time for a diplomatic solution, though he warned "that window is closing."
After Biden's speech, AIPAC - which has not always seen eye-to-eye with the Obama administration - praised him for a "a very important statement today that the president is not bluffing."
Iran will top the agenda on Obama's first presidential visit to Israel, which Biden said would take place just before the Jewish holiday of Passover, beginning on March 25.
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 it's putting itself in a position to cross that line very quickly once it decides to do so." 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.
In Kazakhstan, the United States and five other powers offered Iran modest sanctions relief in return for curbing its most sensitive nuclear work. There was no breakthrough but the sides agreed to further talks in early April.
Netanyahu has insisted that Iran, whose leaders have frequently threatened Israel, is using the negotiations to stall for time to develop a nuclear bomb capability. 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."
Obama has repeatedly pledged to keep pressure on Iran, but his refusal to take an even stronger stance has contributed to tense dealings 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.
Netanyahu also made clear Israel's concern about where Syria's stockpile of chemical weapons and other advanced arms might end up in the midst of civil war.
"As the Syrian regime collapses, the danger of these weapons falling into the hands of terrorist groups is very real. Terror groups such as Hezbollah and al Qaeda are trying to seize these weapons as we speak," he said. "We have a common interest in preventing them from obtaining these deadly weapons." (Editing by Warren Strobel and Christopher Wilson)