By staying outside the 1970 NPT, Israel has not had to forswear nuclear weapons or admit inspectors to facilities, which foreign experts believe have produced plutonium for up to 200 warheads.
The United States has acquiesced to Israel's nuclear opacity, angering many Arabs and Muslims especially in view of Western efforts to curb Iran's uranium enrichment plans.
"To our friends and our allies we say 'there is no room to pressure Israel into signing the nuclear Non-Proliferation Treaty'," Israeli Defence Minister Ehud Barak said in a speech.
"Israel has never threatened to destroy other countries or nations, whereas Iran today, and in the past also Syria, Libya and Iraq that have signed the treaty, have broken it systematically with explicit threats on Israel's existence."
On Tuesday, Obama was asked at the close of a U.S.-hosted nuclear security summit about the prospect of putting pressure on Israel to come clean about its capabilities.
"As far as Israel goes, I'm not going to comment on their programme," he told reporters. "What I'm going to point to is the fact that consistently we have urged all countries to become members of the NPT."
The United Nations holds an NPT review conference next month at which Israel is expected to be singled out for censure. India and Pakistan, both nuclear-armed, have also refused to sign the NPT.
Israel says it would consider entering a deal to rid the Middle East of weapons of mass destruction once there is comprehensive regional peace. Some of its neighbours insist Israel must disarm first.
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