US president Barack Obama has promised to work closely with Israel and do whatever is necessary to keep Iran from obtaining nuclear weapons.
He also pledged to investigate whether chemical weapons were used this week in the neighbouring Syria's two-year-old civil war.
Mr Obama, after meeting with Israeli prime minister Benjamin Netanyahu, said of Iran's nuclear ambitions: "We prefer to resolve this diplomatically and there is still time to do so." But he added that "all options are on the table" if diplomacy falls short.
"The question is, will Iranian leadership seize that opportunity?" he added. The president said Iran's past behaviour indicates that "we can't even trust yet, much less verify."
Mr Netanyahu, at Mr Obama's side for a joint news conference, said that while he appreciated US efforts to thwart Iran's pursuit of nuclear weapons through diplomacy and sanctions, he said those tools "must be augmented by a clear and credible threat of military action".
Although preventing Iran from developing a nuclear weapon is a top priority of both Israel and the US, the two leaders have differed on precisely how to achieve that.
Israel repeatedly has threatened to take military action should Iran appear to be on the verge of obtaining a bomb. The US has pushed for more time to allow diplomacy and economic penalties to run their course, though Mr Obama insists military action is an option.
On another troubling issue in the region, Mr Obama said the US is investigating whether chemical weapons have been deployed in Syria, and he said he was "deeply sceptical" of contentions by Syrian president Bashar Assad's government that rebel forces were behind any such attack.
Both the Assad government and Syrian rebels have accused each other of using chemical weapons in an attack on Tuesday.
Mr Obama said the US policy not to intervene militarily or arm Syrian rebels thus far is based on his desire to solve the problem with world partners.