VIENNA, Sept 16 (Reuters) - Australia, Britain and the United States have informed the U.N. atomic watchdog of their new security partnership that will help Australia acquire nuclear submarines, and both sides plan to "engage" over the coming months, the watchdog said on Thursday. The International Atomic Energy Agency is tasked with keeping track of all nuclear material in countries that, like Australia, have ratified the nuclear Non-Proliferation Treaty (NPT) so as to make sure none of it is being siphoned off for use in a nuclear bomb - an area of IAEA work known as safeguards. So far, only the five nuclear weapons states recognised by the NPT - the United States, Russia, China, France and Britain - plus India, which has not signed the NPT, have deployed nuclear-powered submarines. For a party to the NPT other than the so-called P5 to have nuclear submarines poses a challenge because they are military vessels that are designed to be undetectable and would often be beyond the reach of IAEA inspectors. It is, however, possible in principle to temporarily exclude submarine reactor fuel from IAEA safeguards if a prior agreement is reached with the body. The IAEA said in a statement the trio had informed it "that a critical objective of this cooperation will be to maintain "the strength of both the nuclear non-proliferation regime and Australia"s exemplary non-proliferation credentials" and that they will be "engaging with the IAEA throughout the coming months"." read more "The three countries have informed the IAEA at an early stage on this development. The IAEA will engage with them on this matter in line with its statutory mandate, and in accordance with their respective safeguards agreements with the Agency," it added.