Israel is widely believed to possess nuclear weapons and to be the sixth country in the world to have developed them, allegedly having built its first nuclear weapon in December 1966. It is one of four nuclear-armed countries not recognized as a Nuclear Weapons State by the Nuclear Non-Proliferation Treaty (NPT), the others being India, Pakistan and North Korea. Israel maintains a policy known as "nuclear ambiguity" (also known as "nuclear opacity"). Israel has never officially admitted to having nuclear weapons, instead repeating over the years that it would not be the first country to "introduce" nuclear weapons to the Middle East, leaving ambiguity as to whether it means it will not create, will not disclose, will not make first use of the weapons or possibly some other interpretation of the phrase. The "not be the first" formulation goes back to the Eshkol-Comer (sic) memorandum of understanding made between Israel and the United States on March 10, 1965, which contained Israel's written assurance for the first time that it would not be the first to introduce nuclear weapons in the Middle East. Israel has refused to sign the NPT despite international pressure to do so, and has stated that signing the NPT would be contrary to its national security interests.